I bought a Peavey Ultra Plus about 6 months ago with blown preamp tubes, bought the cheapest 12AX7s I could find just to get it up and running, and I feel that it's hurting the tone potential of the amp.

I was thinking a JJ ECC803s in V1 and ECC83s in the rest of the slots, but I heard that the JJ preamp tubes are dark, and this is a dark amp as it is. Any recommendation here would be great.

There are some cheap Ruby 6L6GCMSTRs that came with the amp in the power section, which I heard suck as well. I have no clue how old they are either. I would go all JJ, but seeing how this is a pretty dark amp, I'm kind of hesitant to go for it.
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JJs are good tubes.

I wouldn't describe the Ultra Plus as dark at all. It's definitely way closer to a XXX than a 5150. What tubes are in it now?

Also just my $0.02, some people disagree so take it as you will: tubes don't make a whole lot of difference. Not to imply that they don't make an audible difference, they do, but at the end of the day, any differences you can get out of different tubes can be obliterated by the difference you can make with a simple EQ.
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I got bored one day and swapped both the power and preamp Mesa Boogie branded tubes out of my Dual Rec and put in my Ultra Plus.
I absolutely loved it so much that I put the Dual Rec on the shelf and solely used the Ultra Plus for nearly two years.
I can't remember what the Mesa's are, but, to me, it's definitely worth checking into.
Try a new Mullard 12ax7 in V1 and JJecc83 in the rest of the pre-amp. JJ 6L6 are a good power tube, but the Rubys you have in the amp are not the worst you can get
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