No, you don't need expensive tubes, you need an
over-sized power supply that is not sagging upon demand. The problem with many power supplies in tube amps is that they simply cannot supply under stress (dynamic peaks etc.) the steady, constant very-high-voltage DC that the tubes need. In many amps the tubes simply underperform
because they never are able to operate under optimal conditions. In my system I have external BorderPatrol MB power supplies, monstrous units for the tube power they need to supply (just 15 W/channel):
http://www.whatsbestforum.com/showthread.php?17334-My-minimonitor-subwoofer-system
The thread also shows the inside of one of the power supplies (they stand on the floor next to the equipment rack).