Retubing power amp

Does it sound good? If yes, then leave the tubes alone. You may want to invest in a tube tester or find a local shop that will test them for you.
If the sound is muddy or the bass is breaking up, then you may be due for new tubes? Also, do the tubes flare when you turn on the amp (give a quick bright flash before settling down to a blue or orange glow).

Since you are not experienced with tubes you definitely need expert help. Spend a little time reading on Audio Asylum. They discuss who to order tubes from, and the symptoms of tube decay. Naturally, the manufacturer will be happy to sell you a new set of tubes, but be prepared to pay double for what the same set can be had for from other suppliers. I can recommend two reliable suppliers.
Antique Radio Supply, for tubes in general, especially rare numbers. Jim McShane for expert matching and advice on power amp output tubes. I have had good service from others, but these two have been outstanding.

You will, after some use, need new tubes for the amp. So, buy a new set now, regardless of whether or not the current set is worn out. Once in a while (say every six months or so), replace the current set with the new set and compare the sound. When the difference is in favor of the newer set to the point that YOU think it is worth the cost of replacing the tubes, make the switch and buy another set of backup tubes. You should also test the power (output) tube set separately from the smaller signal/driver tubes since the power tubes tend to require replacement at an earlier date. Having extra tubes will also mean that you have a backup for the occasional premature death of a tube.

Rod,
I've owned a BAT VK-75SE, and a highly modified one at that. As with most BAT gear, there is not much you can do with tube rolling, as the tubes used are all based from the Russian military. You could try spending $300/tube for BAT's NOS 6H30's....you may, or may not hear a difference.

I spent big $$$ trying to replace the (6) 6SN7's against the advice of Victor Khemenko. The NOS red base Sylvania's did sound better, IMHO....but after 2 weeks they broke up and stopped working. I put the stock Sovtek's back in and the amp worked as normal. Victor warned that the VK-75SE amp pushed the 6SN7 tubes very hard....I didn't listen originally, but after time I would agree with Victor.

Cheers,
John

Ditto Samujohn's referal to Jim McShane, from whom I've purchased Tung Sol reissue 6SN7 & 6V6, and Sovtek 6H30pi for VK75SE. NOS 6H30-DR are NOT worth the $$$. Buy 6C33C-B in bulk from cheap eBay Eastern European sources.