The most popular have been Mullard CV4004 for years. There was a period where the Brimar CV4004 was hot, but both are great. The Brimar are warmer on top slightly.
i use new production yogo's. el12ax7s, nickle plates. very very clean highs. in another application they were a bit shy in the lower mid range but i can't hear this effect on my alpha. i've never tried the NOS tubes recommended by upscale audio. these can be purchased on the internet for a low as 6.95 each.
You should give some NOS 5751 tubes a whirl, which are often used as a 12AX7 substitute. I'm using them in my Norh CD-1 and have had great results - more relaxed sound, less high frequency grain, better detail. These effects are subtle, but definitely there. Check out Joe's Tube Lore on the Audio Asylum FAQ section for more info and specific tube recommendations.
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