I have a Peavey Predator American made Strat clone that my brother in law gave me to get started. He also gave me a 15 watt Peavey amp.
I've been playing for about 4 months now, daily practice for 30 to 60 minutes.
I noticed last week that when playing on the bridge pickup, the G,B and high E strings sounded very tinny, almost banjo-like. And when strumming an open G chord, the low E string was EXTREMELY bassy. I adjusted the amp settings, but it still sounded off.
Last week I took the guitar in for a set-up and it sounded great for a while, but yesterday started doing the same acting-up. The guitar tech lowered the action as well, but I noticed the action popped back up to where it was before I brought it in. I picked up the amp to see if there were any loose wires or something, only to hear something rattling around inside. The reverb never worked, so I opened the amp and found the reverb springs were loose inside.
I bought a Roland 40XL amp today, figuring my troubles would be over. The guitar sounds a little better, the new amp is very responsive to adjustments, but I have the bass set at 2 while mid and treble are at 5. That open G chord still sounds horrible, the low E string very bassy.
Could it be the pick ups? Could it be the acoustics in the room?
I am completely perplexed. The Peavey is about 20 years old.

I'd say it's something with the guitar. Sight unseen though, we could be guessing forever. However, what immediately came to mind while reading your symptoms is that these 'offending' strings are too close to their polepieces of one or more of your pickups. Another 'flag' here which may or may not be significant to your immediate complaint is when you said the action had popped back up after being adjusted. Clearly, there is some kind of instability going on with your guitar here. Having said this and after your tech having set up the guitar a week ago, I'd bring it back to him, explaining what you've explained here. If it baffles your tech and/or he wants to charge you again, get another tech pronto.

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You dig?

Thanks, Rock Star.

Any time!

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You dig?

I agree that the pickup height might be part of the tone issue. try adjusting the height on the top and bottom to see if you find a better sound. (Note, if you have a floating trem. you can get the pickup too close and it can pull on the strings and keep your tuning out of whack.)
As far as the action, it almost sounds like a truss rod issue. At it's age the rod could be damaged or broken. If you can play a chord like your G and apply pressure like you're gonna fold the head to touch the body and you hear the strings lower like a tremelo than you need to get that truss rod looked after. The only other thing I can think of is where the neck joins the body if it bolts on.
Unless, you are using super heavy strings which might be pulling on the neck too hard.
It's too soon after the set up for there to be issues. So go see your tech. I hope it's someone you really trust otherwise he might blame the guitar instead of admit he missed something.

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TOO MANY GUITARS, TOO LITTLE TIME!

Current Stable:
Fender Stratocaster MIM 07
Squire II Stratocaster MIK 89?
ESP LTD MH-100QM
Jackson KE3 Kelly MIJ

Many thanks. I plan to bring it back today to see what he says.