Each A-S amp has one or two 6SN7 positions with the largest impact on its sonics; those positions vary by amp and vintage - email A-S or check the OTL forum on the Asylum to locate this/these on your amp. You will get the biggest dividends there and the differences are not subtle, at least on acoustic music. Of course other positions also affect sound, just not as much. There is, imo, no modern 6SN7 the sonic equal of a quality quiet NOS valve in A-S gear. Agree with Pubul57 about Andy at Vintage Tube Services - he takes his time but excellent product - best to call rather than write.
Here is a good place to start.
My NOS favorites:
1. Sylvania VT231 & 6SN7GT (especially the '52 green label w/ 3 holes in each plate)
2. Ken-Rad VT231 & 6SN7GT
3. RCA VT231 grey glass
4. Raytheon VT231
The current Shuguang CV181-Z Treasure are ok but also pricey - I found them soft in the bass compared to any of the NOS above. The Electro Harmonix 6SN7 are ok: slightly soft and warm, kinda like a Sylvania GTB, but the latter will last longer and its easier find quiet tubes. The modern standard Shuguang 6SN7GTs (not Treasure) that come stock with the amp are consistent, reliable and quiet, but sound rather mechanical to my ears when compared to NOS, and do not last as long. Stick with the stock Russian 6AS7.
Here is a good place to start.
My NOS favorites:
1. Sylvania VT231 & 6SN7GT (especially the '52 green label w/ 3 holes in each plate)
2. Ken-Rad VT231 & 6SN7GT
3. RCA VT231 grey glass
4. Raytheon VT231
The current Shuguang CV181-Z Treasure are ok but also pricey - I found them soft in the bass compared to any of the NOS above. The Electro Harmonix 6SN7 are ok: slightly soft and warm, kinda like a Sylvania GTB, but the latter will last longer and its easier find quiet tubes. The modern standard Shuguang 6SN7GTs (not Treasure) that come stock with the amp are consistent, reliable and quiet, but sound rather mechanical to my ears when compared to NOS, and do not last as long. Stick with the stock Russian 6AS7.