As a gigging double-bassist, a combo amplifier has been a must as long as I've been playing. In most (non-classical) situations, I have to have an amp, and preferably one that can reproduce the sound that I'm making on my bass with volume, clarity, and bottom. Most of the time, I'm looking for an amp to give me that "bump" in volume and presence so that I can be heard in a group that might have drums and keys and guitar. That sound coming from the amp, combined with the sound coming off of the instrument is a beautiful thing. Sometimes, though, I might have to play with a big band and I need an amp that will allow me to be heard in a large ensemble with focus and bottom without feeding back. Fast forward to a couple of years ago. I started using the trusty GK, like many of us. It seemed to faithfully give me what I expected, especially when it came to that "bump" I needed on most of my gigs. It's a lightweight amp, totally cartable, and reasonably loud. I also found that it sounded best (to my ears) up off of the floor in some way. It seemed to work for my bass run mostly flat, that is, the knobs facing straight up, with slight bass boost and hi-mid at times, and often cutting the treble a bit. I play a pre-WWII 7/8th's (most likely) Juzek with Sprio Mittels and a Helicore Hybrid Med 'G'. As "jazz" as they come. It's a loud instrument that I try to give just a bit of help with an amplifier. With my Realist pickup, this setup worked for me. Most often, for club gigs, and in situations where I didn't need to turn up my volume that much, this was a usable sound and I was mostly content with this setup. Then, it seemed as though I needed to be louder for a series of gigs. Last of which being a jazz mass I played recently (played with full choir in concert). The GK just seemed not to do it for me. I was overplaying, kind of struggling to hear my notes, and not enjoying the kind of tinny sound that the GK was prone to putting out at higher volumes. I tried adding another cabinet (a Markbass 102p) along with the MB150s, but soon realized that I was only amplifying a sound that I didn't like at high volumes! I tried a preamp in my chain, to no avail, even bypassing the preamp on the combo itself a la the effects loop (thanks, talkbass). It wasn't working for me. It still seemed to feel like I was playing bass in the next room. I was yanking the instrument to get any kind of presence or punch--especially when the volume went up. I needed something different, especially when my bass sounded so damn good when I was playing it acoustically. Enter the Markbass 121p. I did a little research and found that this might be an amp that I would want to check out. I actually own the LMK II and have always loved the sound that it put out on my electric bass--but not so much with my DB. Still, it intrigued me enough to call around to see if GC had one sitting around that I could try out and play through a bit. I got lucky and actually found one that was used in new condition. I hurried over to try it out, and was pleased with what I heard. If only every gig I did was at GC. Thank God for the 14-day return policy. I was off to the house to do an honest A-B test between these 2 amps. Something I haven't seen on here as of yet. A-B TEST As I said, my DB is pre-WWII, fully carved, with a Realist (soon to be FC again--it's getting fixed) and Spiro Mittels on the E-A-D with a Helicore Hybrid Med 'G'. I plugged both amps in and lined them up, about 2 feet apart, facing me. Both gain stages were set and EQs were set flat. I tried to find an appropriate volume level that was equal between the two amps. Here's what I noticed: The 121p is a loud little amp, but the GK has much more bottom. By that, I mean SUB frequencies. The sound of the GK seems a little more complex, especially around 40Hz and lower. Mind you, I could really hear these low lows playing by myself at home. The Markbass, set flat, can only be described as much more ORGANIC sounding, with low-mids that jump out of the box. The sound breathes much more with the 121p. It blossoms, whereas the GK is much tighter sounding. Turning up the 121p opens up the sound a bit, and the punch that it puts out is undeniable. Adding more volume on the GK definitely adds a much more metallic-type edge to the sound. Next to the Markbass combo, it definitely sounded on the 'sterile' side. I tried then to add a bit of EQ to each amp. The GK's EQ was much more fast-acting. Minute adjustments seemed to have a much more drastic effect on the sound coming out. I was surprised that the GK really didn't lack for sub-bottom, especially when engaging the bass EQ, but it was a very 'tinny' kind of sound that just increased as volume increased. The controls on the 121p seemed much more subtle when first engaged. Buttery would be a good word. Not as drastic as the GK's controls. It seemed to me that the EQ on the 121p was much more "usable" than the GK's. By that, I mean that you could tame certain frequencies and still have your sound, whereas when you mess with the MB150's EQ, you're making much more drastic changes. When it came down to it, the GK's sound seemed much more "complex" than the 121p's, but the Markbass definitely had a sound that I liked a lot more, and I wanted to see what that sound was like in a real-life gig. I took the Markbass combo onto a gig in a small theater, about a 200-seater. The group was bass, drums, piano, sax, and vocals. On a small stage, I plugged into the system with an XLR (and later the soundman told me that he really didn't need to turn me up in the house--the amp did well enough). I turned on the amp and proceeded to warm up. I had the 121p set mostly flat, with maybe a tad bit of HI-MID boosted. I was not used to hearing that much punch come out of my amp, and clarity--especially with a Realist. With the wrong amp, a Realist can really get lost, but at times on the gig the amp seemed to disappear--especially on the 2nd set. I messed around with the volume, as well, and I much preferred the headroom that 300 watts was giving me. Playing through a ported enclosure was also a step up, soundwise. I placed the amp on the floor next to me (in between myself and the drummer) and I've never been as happy hearing my sound come out of an amplifier. I got the "bump" that I love to get, but I also got the punch coming out of my instrument, the sound was much quicker, and much less transient. Everything I played was right there in my face, not in the next room. The 'tinnyness' was gone, and while the bottom was not as complex, the much more organic sound blended with the group much better than that of the GK. There was a low-mid presence that had never been there before that was a delight. I also felt like I could turn WAY up if I had to and not really lose that sound. I could drive a big (loud) band and still have great, punchy, present tone. Of course, I'm anxious for the next gig, and the next, but I had to post my initial impressions. This is a great amp, and it's going to replace my MB150S when it comes to my DB. While it may not have the kind of complex bottom the GK puts out, its just as transportable, its sound sits even better in a mix with other musicians, and it plays much faster and punchier without sounding 'tinny'. It's also loaded with more power and can be pushed alot further volume-wise than the GK. Of course, YMMV, but I am very impressed. . . .