Yeah..like others have mentioned my first thought was to try a better phono preamp. But then if the Pro-ject makes you happy & seems to "synergize" with your gear I guess that's a good thing.
I'm kinda jealous that you live so close to a tube hoarder but not jealous of those prices. Legit tubes are becoming harder to acquire though. I've always struggled with paying for tubes that cost anywhere close to the price of the actual component used. If you start DIY-ing that situation comes more into play.
If some of these tubes are truly unused you will experience some changes in sound & perception from them. I have definitely experienced tubes that sounded better right out of the box...strong emphasis on bass & vocals for them to eventually even out. Where I actually preferred them when they weren't so linear sounding. But the same could be said that they sound kinda Meh at first & eventually transform into something more liquid. My point being that you really need to put some time on them before realizing their true nature.
My approach would be to get a pair & wait a few weeks before trying something else. If you sample a pair that you really like & they are over $200 a piece I'd remember the exact tube type & at least explore elsewhere for a better price.
Expensive doesn't always mean better. Just like with your more modest prices preamp, you may experience that a $50 pair of tubes does just as much magic for you.
I wish I had a nearby neighbor that was cool enough to let me sample tubes. You are extremely lucky. I've spent good money on tubes that definitely were not worth the price I paid. If you decide to pay big dollar prices make sure these tubes are tested appropriately. Find some everyday tubes & then tubes for special occasions. Get some "Socket Savers". Eventually getting socket savers for all your favorite tubes..leaving them attached to your most expensive favorite tubes 24/7. Just my 2 cents. It blows my mind how expensive tubes are..but I guess back in the day I thought they would be around forever. Then I got into surround sound & solid state & didn't think about tubes for 15 or 20 years.