I own a 3A and I use NOS Mullard tubes that have lasted for years. The 3A is detailed. If you want the tube sound, you can't beat the older CJ's; PV10, PV11, or PV12.
What's the rest of your kit? The 3a may be leaving money on the table if the rest of the gear doesn't jive, or depending on how you listen you made opt for the 3...
I've owned them all and currently have a 3b (just back from Art) with his lastest gen caps.
But sorry I don't know the details of the upgrade, but with the right gear it's well worth the extra $$$.
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