My favorites are:
2 - Svetlana 6S19P-V or 6S19P-B (same tube)
2 - Ulynovsk 6S19P-V or 6S19P-B (same tube)
2 - Tung Sol 6AK5 or 5654
In exchange for Tung Sols - others like in about this order of preference:
2 - Western Electric 403B
2 - Mullard 8100 / EF95
2 - RCA 6AK5 or 5654
2 - Sylvania 6AK5 or 5654
The Tung Sols are a bit hard to find - but they're really great - fast, solid, clear.
The stock tubes aren't terrible.
My only problem is I have too many WE403Bs (around 12 tubes) but can't find any extra LM403B. I don't know why I can't stop bidding those WE403B when seeing it on eBay, though I don't really prefer its sound.