Re: Preamp tube question
Well, of course the glib answer is... it depends, and, do whatever you think sounds best!
I know that when Joe Satriani was developing the JSX amp he made sure that it worked well with the effects he would put in front of it, tubescreamer, whammy etc. The inference being that some amps, particularly high gain amps, don't respond too well to having additional distortion/overdrive plugged in the front.
Interestingly on a G3 video I saw, Steve Vai and Joe Satriani were discussing this exact issue and commenting on the relative benefits of the tube screamer and boss DS-1 distortion. Steve Vai is working on a combined dual distortion/overdrive pedal with Ibanez at the moment called the jemini.
From my experience, for the last decade I have never used an overdrive pedal or the overdrive section of a multi effects. I always use the valve distortion from my Laney VH100R amp. I always thought this was how it should be done, but *most* people seem to use an overdrive/distortion pedal in front.
I have a GT-8 multi effects which I wire up using the "4 cable method" this means that the signal goes from the guitar to the front of the GT-8, GT-8 send to the front of the amp, amp send to the return of the GT-8, GT-8 output to the return of the amp.
This means some GT-8 effects can be in front of the amp e.g. Wah, compression, then you get the Laney pre-amp, then you get delay, chorus, reverb etc. after the pre-amp.
As you can move the location of the send and return in the effects chain on the GT-8, you can change which effects are pre or post amp pre-amp.
As regards the power amp valves. I changed from 6L6s to EL34s years ago and have never changed back, although in honesty I can't remember the sound being bad before!
'91 Jem77BFP with GK-3
Charvel EVH Art Series (red/white/black)
Burns Brian May
Epiphone G-1275 double neck
Epiphone Sheraton II