I played one of these for a half hour today at OKC Music and Sound while I was having a jack repaired on an instrument. I played an old fretless MIA Peavey (a foundation from 83 I think) through the Streamliner and into a Mesa 210. What follows is my long winded and probably unnecessary thoughts on the head. The guys at the store said they just got it in that day, so I figured my ramblings might benefit someone.
The power up delay/warm up felt like longer than the stated 20 secs. Felt more like a solid minute. Not a big deal either way, and to be honest I wasn't exactly watching a clock on power up. Might have been just me being impatient.
Without engaging the gain boost button the sound was fairly clean. Maybe not as clean as my old squeaky clean EA Iamp, but definitely clean. Honestly even though I am a clean amp guy, this setting didn't do much for me. Hitting the gain button fattened up the tone a lot. At first I had way too much overdrive with it. After futzing with the gain structure for a while, I settled on low on the volume and master and relatively high on the gain. This setting was cleaner than I thought it would be. I got more overdrive when raising the volume knob (appears to be an input volume) but ran into preamp clipping issues when doing so. With more time I could probably get the gain structure to yield more overdrive if desired. I was trying to be polite and not be the guy playing obnoxiously loud in the corner in the store, so I didn't go nuts here.
The bass knob felt voiced fairly low. The 210 I was playing through wasn't exactly the lowest extended cab out there, so the low EQ felt like it did a good job at compensating for it. The treble knob did what treble knobs do. Since I was playing a fretless (which normally doesn't sound great with treble boosted), I wasn't in the best situation to test high end sheen. I attenuated the treble slightly (say 11:30) and left it there.
The mid knob I experimented with the most. Boosting the mids to 3 o'clock and having the rotary selector selecting the lowest mid frequency (350 hz I believe) I got a nice thick tone. Really beefed up the perceived low end. Cutting to 9 o'clock just resulted in an anemic tone on that Peavey. Boosting low end to compensate just didn't do it for me. I don't ever see cutting those lower mids. Setting the mid freq switch to the middle position (650 hz?) and boosting to 3 o'clock got a great burp. Favored the bridge pickup and played the obligatory Jaco. Harmonics rang out nicely. Scooping out this freq to 9 o'clock yielded the stereotypical contour sound. Not a useful setting for what I do, especially with a fretless. Perhaps more useful if you are going for a Marcus Miller slap tone. Setting the mid freq to the highest setting (2.5 khz) was pretty interesting. Boosting to much gets twanky pretty fast. On the other hand, since I had the gain relatively high, that frequency boosted interacted with the gain to really bring out the grind. This setting would probably work wonderfully at helping to cut through a denser rock mix. Too much got ice-picky quickly though. Definitely would need to spend some time finding the sweet spot if I wwas boosting this frequency. Cutting this frequency let me roll the tone knob up and not have annoying string noise. If I wasn't using one of the other mid settings already, I'd probably consider leaving the amp set here to help deal with any string clank (as opposed to just playing cleanly

). Overall though this was my least favorite mid freq to play with. Sidenote: Boosting this frequency, 2.5 khz, with the gain turned up really reminded me of ho the presence knob interacts with the drive knob on my SABDDI..
The overall volume this head was putting out could have clearly driven that Mesa 210 harder than would have been wise. Since I was in a store, I refrained from being a jackleg with the master knob. Don't know if it is due to the preamp voicing or not, but it seemed to have more volume on tap than the shuttle 6.0. Most likely that has a lot to do with the high setting on the gain knob I was using.
Comparing this to the Walkabout, which I've never owned but have gigged several times, the streamliner seemed like it had a tighter low end. I also think it will probably stay cleaner down low at high volumes. That opinion is me extrapolating the tone I heard at lower volumes and not direct experimentation. I could be completely wrong there. I really like the walkabout but this amp seemed like it was voiced more towards my tastes. The walkabout always felt like it was a touch more overdriven when played loudly than I wanted it to be regardless of how I set it.