The good news is that the amp runs only four tubes. That'll keep cost down if you want to start experimenting. I'd say run them with the stock tubes to familiarize yourself with the sound and it'll be easier to discern any differences once you start "rolling."

Aargh, you're gonna make me want to start looking at headphone amps again. Must stay away from head-fi. . .

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"Nothin' up my sleeve. . ." --Bullwinkle J. Moose