...if you have to for example boost the mids past 3 oclock or cut bass past 9 o'clock to find your tone generally or close to those extremes. You are then left with very little wiggle room when you find yourself in a room that needs adjustment. Mids become cut only and bass boost only.
I generally agree, experiment, twist knobs and find your tone. But when you start to hit the extremes you have to start to think if it is really the right tool for the job.
I agree with you in principle, because I prefer it when amps are voiced so that they're flat with the controls at noon. I prefer to be able to see my EQ curve, just by glancing at the knobs. I even cared enough to write agedhorse, to see if the preshape could be changed.
However, the Streamliner is challenging my thinking, for three reasons:
1.) There is still a reasonable amount of bass cut available when the knob is at 9'oclock, and ditto for mid boost from 3 o'clock.
2.) It has only taken me a short while to get used to the idea that 9 o'clock and 3 o'clock are "the new noon".
3.) It sounds so damn good.
Besides, how often does anyone reduce the bass below flat on the gig? The only time I only ever cut the low end below noon on ANY amp, is because it has a pre-shape, and I'm trying to get it back to flat. Just like I do on the Streamliner. The same goes for boosting the mids. On most amps, I tend to run the mids higher than other people. But that's just because I like less mid scoop than the norm, not because I need 10 or 12 db of actual mid boost above flat.
Unless you've got a strangely-voiced bass or cabs, I can't really imagine needing more EQ range than the Streamliner offers, at least with electric bass. Unless it would be different with upright?
On the subject of maxing out the EQ to achive the tone that your looking for has not sounded very good with any other equipment I have done that with,it just sounds wierd or picks up hiss or static.
Once again, I totally agree in principle. Almost all amps have a huge amount of range on the tone controls, but in most cases only a small part of that range is actually useable. Big boosts and cuts tend to wreck the tone on other gear.
Except that once again, the Streamliner seems to break all the rules. I may not like every setting on this amp, but to my ear, they're all useable. No objectionable hiss when the treble is dimed, no weird wa-wa-pedal resonances when the mids are pushed...
More to the point, I would be willing to forgive it just about anything, given the fact that it hits my target tone without weighing a ton. I've been searching for that combination for decades.

