Ajax, I'm waiting on a Bluesbreaker (JTM45 w/trem) and I'll be going through this also. You've probably discovered that the Metro Amps forum is THE place for JTM tweaking info.

The impedance selector is standard.

Reportedly, some folks get good results from just properly controlling channel interaction, where even un-jumpered interaction has an effect. The most common mod to reduce flub is lowering the V1 bypass cap. It appears Nik may have slightly lowered it already, since folks at Metro refer to it as 250uF but Nik's layout shows 220uF. Reported mod values vary wildly, from  100uF or 50uF to as little as .67uF.  I'm inclined to think 50uF to 10uF is the sweet spot, based on reading? I'm thinking that if this mod is significant, a switch to select from maybe three values -- stock, reduced and way reduced -- would be nice. From there, you can get off on all kinds of stuff including tube types, rectifiers, split cathode etc. A 12AT7 or 5751 at V3 (PI) can increase headroom, which should add clarity.

Other common tweaks include removing either one or both channel 2 bright caps to tame harshness on that channel (one at the vol pot and one at the 270k mix resistor). The NFB loop can be tweaked in various ways (use a different OT tap, change the feedback resistor value, add a in/out switch or control pot, etc), but reducing NFB can increase flub as it opens overall frequency response including lows and low mids. I had Nik up the screen resistors on mine to 1K 5 watt, which adds reliability with current production tubes and shouldn't really affect tone.

Anyway, read at Metro, but understand the flub issue remains very subjective where some users love stock and others can't stand it! I guess one man's flub is another man's warmth? And some of these guys are dealing with Marshall RI's, where issues like flub and harshness are more severe than with quality clones like Ceriatone, Metro, Germino etc.

So ... any Ceriatone JTM folks with experience tweaking the V1 bypass cap?

i got the jtm45 bluesbreaker from nick.
got a question related to what you just said, just i whant to make sure.

on the layout/amp the pi filtercan cap is a 16x16 and only one side is used.
and the preamp have 16uf of filtering too.

i whant to have 16x16uf filtering to te preamp adding the 10k resistor.
instead to replace the can, for a dual16, can i simply keep the already in single 16uf capacitor for preamp, and use the other side of the pi 16x16 can cap for the preamp or it is better having a dual 16x16 only for the preamp?

i ask this because instead to buy and change the cap it could be easier, and if i dont like it i can remove the change and cost nothing.