I dunno about the 550 fusion, but I've been using JJ tubes from Bob Pletka (Eurotubes) in most of my guitar amps for 15 years. Bob's preamp tubes are graded for distortion characteristics within each series (12AX7 low, medium, and high gain, 12AT7, short plates, etc.) so you can get just about any preamp 'flavor' from a JJ tube that is possible. The only amps I never like JJs in were Mesa Boogies. All my Peaveys, Matchless, Laneys, and one Ceriatone got JJs of one stripe or another.
If my guitar tech called a JJ tube "way too aggressive" I'm finding another tech, because that shows pretty severe ignorance of how preamp tubes in general are classified, sorted, and sold. Not just JJs. Hopefully your tech just meant the SPECIFIC tube in YOUR GK broke up way too early for a bass pre. It's gotta be easy to get to. Pull it out and check the numbers on it. In some guitar amps preamp tubes can vary the tone wildly, in others ... not so much. Some amps just sorta sound like what they sound like, and pre and power tubes just get you an ealrier breakup. Not sure how much variation one can get from bass pres but I'd imagine it'd be noticeable.
Last edited by Phlipper : 02-26-2013 at 04:11 AM.