New Tubes

500 hours for 12AX7's is barely broken in. They are capable of 5000 - 10,000 hours. I had 60's Fisher tube gear with the originals still installed, working and sounding fine.

What he said. However, nothing to stop you rolling the tubes. NOS (or used from good dealer) Telefunkens would be a good place to start.

Most gear today is voiced with JJ or EH, both are good stable valves. I don't think really good NOS valves will sound much better. This unit uses WiMa caps they have a sound more so than the actual valve. Both are very neutral, JJ or EH and Wima. That's my take on what the inside looks like. No fancy resistors, or parts, it's not point to point it's not all crammed together either.

I wouldn't spent too much on valves unless (like me) you use 12AX7 as a main valve in a lot of your gear. I keep good 12AX7s handy.. I personally match my valves. I'm way to picky. But my Macs and Cary's both sound very special because of it..

Blackburn (Mullard)
Telefunken
Amperex
Valvo
Mazda
RCA (only confirmed BP)
Solvex (W) black plate (BP)
GE NOS BP
Tungsram

New Stock

Golden Lyon
PSVANE
JJ
EH

Just about in this order, GE and Solvex can swap spots.

Regards

Of course one can find better sounding tubes and I would too, however I was just addressing the OP calling 500 hour tubes "old tubes". 

op

i don’t know this specific pro-ject phono stage you have...

that been said, i would say, in general, my own experience is rather different than @oldhvymec (who may well be right in what he says referring to your specific unit)

in my experience, the modern russian/chinese/slovak small signal tubes are noticeably inferior in sound quality to even fairly pedestrian old stock equivalents - i am referring to the popular 6dj8, 12ax/u/t7, 5687 families, as well as 6sn7/sl7 octals commonly used in front ends of better tubed hifi gear

to my ear, the modern tubes have good frequency extension and are reliable, but they typically have a little (or more than a little) graininess, and lack the warmth, body, tone, imaging and overall smoothness offered by even modest old stock american ge’s, rca’s sylvania’s tungsol’s (not to mention the european or japanese makes mentioned)

I wouldn't spent too much on valves unless (like me) you use 12AX7 as a main valve in a lot of your gear. I keep good 12AX7s handy.. I personally match my valves. I'm way to picky.

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Then I listed what I use in order. 60.00 and some shipping.. Tungsram they will match 1500.00 Blackburn if you listen.

jjss49, like I said, it is the main valve I use, 12AX7, those valves NEVER leave my sight until they drop dead. :-) I never sell a unit with my personal valves, they will be buried with me.. To BAD!!! :-) Some set are 40 years old now..

I've had some guys offer me a lot of money on a couple set of RCAs I use.
GEs there are some real good ones, Sylvania, not so much. Power valves yes, small valves NO, to darn bright.

The New stock, seldom do I use unless it's just a place holder, like MC, MM tube positions on Macs. As long as they aren't noisy, I'll use 1.00 pairs (no problem) in leu of 500.00 pairs.. 

I've paid 50 cents for Russian surplus that were as good as any EH 12ax7 at 20.00 a valve. I just had to buy 2000 to get the deal. 4 times I purchased them before stocks ran low and prices X10. Surplus Solvex 1950-70. great valves.

Regards

Thank you for all your knowledge and help. 
I will listen for a while before deciding.