Locating capacitors

Make sure the amp has been off and you know the high voltage power supplies have been bled. Otherwise you might be in for the shock of your life. I also suggest you get caps rated at 105C. The Panasonic's are quite good. The bigger issue for radials in lead spacing. Make sure you get that close or the cap will not sit flush up against the circuit board.

It doesn't need to be a Nichicon. There are plenty of good quality cap manufacturers out there. Why don't you call the manufacturer of the tube amp you own and see how much they want for the part.