Because at the time the IRF is all i could easily get. But your right the IRL is supposed to be slightly better. The difference isnt to do with output volume but the quality of what you hear, with the 510 it has much more clarity and detail. I think its a faster part which would translate into better resolution.
On a side note i picked up a Sylvania and GE tube for it which i tested out last night. Oddly i think i prefer the Chinese tube that came with it
@money4me247: There is only one way to learn, no better place to start than these budget amps
Interesting. I guess if I wasn't so busy at work, I might go for the IRF. I might still, so thank you for that pointer
The Sylvania and GE 12AU7 tubes I have tried, are only marginally better than the stock Shuguang tubes. The GE 5-star 5963 was noticeably better. Long black plate tubes, even better than that. Of the old American tubes, I'd have to say the Tung-Sol tubes are probably tops. You need to go to Amperex, Telefunken, or Mullard tubes to improve over those. The quality among those three European makers is tops; the only choice is whether you're looking to brighten something up, or tone it down.
I had to distract myself from some family issues over last summer, so I did what some might consider foolish, and started collecting 12AU7 tubes (and its plug-in variants like 5963's and 5814's). And rolling them in my stock Bravo V2 I left some tube rolling notes in this thread, along with another member's impressions. That may help you decide, should you continue that effort.