#1
I just picked up a peavey ultra plus off craigslist, and it sound great except for the bass. Some times the bass sounds like a fuzz pedal kinda thing, and I think it might be the pre amp tubes. They are at least 7 years old, mabye even 11, and i just replaced the power amp tube with JJ 6l6.

Should I get new pre amp tubes before trying anything else?
bugera 333xl (for sale)
B-52 4x12 cab
Schecter Hellraiser Avenger
Schecter c-1 FR prototype (for sale)
Squier strat

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#2
Congrats on your purchase and ABSOLUTELY you should get new preamp tubes. They are relatively cheap and easy to replace. I personally would get a mix of tubes depending on your budget. A JJ, a Tung-Sol, a Shuguang, a Sovtek LPS, a Mullard, a JAN Phillips 5751, a Penta Labs 9th gen, go crazy. If you like a dark tone you can't go wrong with all JJs. If you like a brighter tone get Tung Sols or Sovteks.

If you are in the States send an email to Doug at Dougstubes.com and get his req.
#3
Congrats! The ultra's are killer amps for the money. I'd definitely get some new tubes. I run a JJ in V1 and V4, and Tung Sols in V3 and V4 of my XXX with great results.