How are Mullards supposed to sound?

The easiest and cheapest way to solve your problem would be to contact Jim McShane or Kevin Deal. They are tube dealers who are known for their knowledge and integrity. Kevin Deal sells the Prima Luna amps and has access to more NOS tubes (I think).

Either one will discuss with you what kind of sound you would like to hear from your amp and will suggest tubes that should get you there.

I suspect that Kevin has dealt with many people who like to get his advice or a demo unit and then buy elsewhere. I'd guess he's gotten very good at detecting these people in a brief conversation. If he's wrong some of the time, that's just the price he pays for not having to spend the whole day on the phone with people who just want to chat. If he passes you off to someone else it's probably someone he thinks is perfectly capable of helping you.

I'm not implying anything about anyone on this thread. You may have had every intention of buying from Upscale.

Running a business is incredibly time consuming, especially a business like high-end audio. I'm sure Kevin could put in 16 hours a day and still have plenty left to do if he didn't keep his attention focused on just the tasks that require his personal attention. Or he may be a terrible guy. I don't know him personally.

Layman, yes tube rolling is a roll of the dice, especially if you are buying tubes say on Ebay. Those tubes that look like they just came off the production line may be all used up.

It takes a lot of study, purchasing a tube tester (another adventure)and quite a few bad purchases to become knowledgeable about tubes as you have found out buying new production Mullards that you thought were the same as new old stock Mullards.

I think the best way to go for people new to tubes is to find a reputable dealer and take his advice.