How far over the limit can I go

Keep going until it doesn’t sound right! Enjoy it. 

Just call Upscale Audio, a very big dealer in bottles (tubes), and buy the bottles from him.  If you've ever wanted to tube roll, here is your chance to have a bit of audio fun.  Next, buy a multimeter along with a small screwdriver, and bias the thing yourself.  

One thing you absolutely need to keep in mind is to replace the 6550 power rectifier tube at 2000 regardless of whether you remove the 8 power tubes.  If you don’t and that tube fails (you won’t hear anything in terms of sound because it’s not in the signal pathway) you can fry the board.  And that is a very expensive repair......

I have a different amp , but tubes none the less . About 2 years ago I got back into vinyl , so I put a fresh set of KT-120’s into my amp. ( auto bias ). The current ones had plenty of life , but after warming up the new ones, there was a noticeable sonic improvement. I purchased them with matching and burn in . So I just let them warm up a couple hours and listen . I have some of the exotic NOS tubes like Tungsol 3 hole , GEC KT-66 and KT-88. But I love the Winged C tubes ! As far as failure , a brand new current production GL KT-77 failed and damaged my amp . Had to send it in for repairs. A capacitor blew with such force that it snapped the junction tab in two, as it’s a point to point unit . Heard a huge pop on my left speaker , but no damage . One of the Winged C’s let go and the glass fractured . The tube internals turned snow white, but no damage . Fortunately, I was standing in front of my system and turned the amp off . Ah the things we do for Harmonic Distortion! Cheers , Mike.

You will have to have the tubes tested to see how they are but tube sound degradation happens over a period of time slow loss until the tubes fail so you do not really notice until you have to fix the tubes because of noise issues or replacement because of total failure.