To the OP; if you choose to use SWR I highly recomend the SM400 or 500's.
Also, they are completely useless without a HPF.
Bought an SM400 is the late 80's and have had it ever since.
Most of it's life has been in a 3U rack with a vented top panel and a fan mounted in the back of the case blowing over the cooling vents.
My buddy has one too and bought it at the same time; no rack / no fan. Bullet proof for both of us; his went down a flight of stairs...
With a HPF and a fan it's one of my main gigging amps.
Just bought a mint SM500 (that did need a tube but otherwise perfect) for $250 shipped. Probably a dumb idea but it was so cheap...
Are the fans in the 500's a little noisy or am I just the lucky one?
Now, when I used SM400s, we didn't have readily available HPF units. Well, maybe we did. But I wasn't aware. There was no internet like it is today, none of my friends used em, I never read about any pro using them. Not that you shouldn't, not that you could, but the SM-400 came with the tools to simulate one. And after a friend had an accident with a Hartke 410xl, totally operator error, I read that manual carefully. Here's what it said, and what I did...
In the original manual, and the one you can download now, it gives a ton of instruction, warning folk of its frequency response, and directions on how to tune it to the cabinet you were using. The manual, of course, says to use SWR Cabinets, but this procedure taught me a bunch about how to use eq, and I use the same procedure to this day with these amps. I personally never had an issue controlling lows. Never blew a speaker. And haven't had to add a Pass Filter in any rig.
Turn up the amp to a listenable volume. Turn the left most Parametric knob fully counterclockwise. Pull the slider above the leftmost knob all the way down. Play a note, lowest preferably, and let it sustain. While the slider, level control, is in the fully cut position, slowly turn the knob, frequency select, rotating it clockwise. At some point in the first third of it's range, the lows will disappear from the cabinet. That is, essentially, the cutoff point of the cabinet's low frequency response. Turning the knob back counter clockwise, stop when the lows partially return, or, like I learned to do, leave the knob where the lows disappear. Now, hit that same note again. And slowly raise the slider and stop when the lows return. I kept the frequency select knob at that point. Always. Each cab is different, but once I found that spot for any cab, that's where I'd leave it. The slider that controlled that frequency level is what I used to tame the sub lows the 400, 400s and 500 fed my cabinets. At home, the slider was near flat, tho later I learned to appreciate the tone with varying degrees of cut. It tightened the response, accentuating the mids, where the tone is, which I tuned with the exact same procedure. The right most slider, the highs, I set just below the cabinets crossover frequency for the horn, taming it, too.
At gigs or rehearsals, higher volumes, I cut the low slider a lot more, til at the volume I desired the speakers quit moving. Or nearly did. This made my rig more efficient, using lots less power to try an reproduce sub lows that were damaging and, that a casual listener would not miss, plus, those lows were in P.A. subs already. I didn't need em on stage. I could hear myself better, thus my volume settings reduced. And, it left me three eq bands, plus bass, treble and aural enhance controls, to manipulate tones or rooms. I could run my gain higher, too. Gettin passive basses to respond like active. Along with the pretty good limiter circuit, this took care of low end issues, and no more blown drivers. With no hpf.
Then I got ideas to steal other amp's tones, in fun. I'd research GK's eq points on their 4 band eq, set my eq to match, and tried their manual's eq settings. The amp taught me a ton about eq, learning what frequencies sounded like, trained my ear, and I applied these things to P.A. eq and such, too.
My friends and I always found the 400, 400s, and 500 a bit lacking in output in our Hard Rock context. That's why I moved up to the SM900. It's response and reactions are quite different than the 400 series. More power, less low end initially, and just a different feel. Where I felt the 400 was lacking for me, the 900 made up for it, in spades.
I still say the SM400 preamp is my favorite sounding. Even Marcus Miller used it's preamp in front of a Bass 350, because the tone was in one, and the focused power was in the other. Today, my heavy rock buddies still aren't keen on the 400, 400s or 500. I play different styles today, and bridged into an 8ohm cab, it's plenty on all but my biggest gigs. Bridged at 4ohms with two cabs? It sounds great, even if the master is up above noon. And it is plenty for anything then.
I still like the 900 for gig use. Bridged at 8 with any cab, it just works better at lower settings for me. But the preamp in the 400 series has a thing. And that amp means a lot to me for the education. I figure all the guys who know SWR know this procedure. Just a reminder for others to read the manuals. Those old SWR manuals are some of the best. And the information in there isn't only applicable to SWR amps. Or just bass amps. It's applicable toward sound in general. PA, too. Even monitors. If you understand the cabinets frequency response, you can use eq to get the most out of any cab. Or system.
Today, filters are everywhere. Easy. Back when we had no clue. These procedures will work for the SM-1500, too. It's tools are far more precise than the old SWR stuff. Cutting lows at higher volumes is usually good practice. Helps in lots of ways. Be it a filter or the provided eq. Being able to select the right frequency for the cabinet can really be an asset.
Also, a buddy taught me a trick with the Aural Enhance circuit. I hit a harmonic at the 12th fret of the A string. And start turning the Aural Enhance up. As soon as I hear it take effect and change the tone, I leave it. Forever.
