Additional Preamp Tubes

On Ebay, I recommend this tube seller.     He’s one of the few who allows returns.  Most don’t.  I returned a pair once – no problem at all.    Probably best to narrow down your choice first.

My DAC uses the 12AU7 (or 5814A, 6189, etc).   IME, I have yet to find a warm variant.  The only exception was the Mullard 12AU7.  My observations were the same as @buellrider97.    Dull and lifeless.  Very unimpressed.   No experience with the CV4003.

The Amperex 7316 is a very nice possibility – relatively neutral with excellent resolution.  On the pricey side.

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Another approach. If you have Synergistic Research dealer nearby, ask if they’ll lend you a pair of upper-tier IC’s or a digital cable with the Red tuning module(s).   The Red warms up the SQ w/o sacrificing resolution.   Good luck!

Recently, I worked with a tube vendor who had a good sense of which tube manufacturer (and which era for each company) had a certain kind of sound. Emphasis on detail (Telefunken, for example), or a sense of warmth (Mullards). When I described what I had, and how I wanted the sound to change, he suggest 1960's RCA's. So, I tried a set of four for my phono preamp - that was the Goldilocks tube for me. I placed them in my three Zesto components  - phono, pre amp, and power amp. I had tried both Telefunken and Mullard in these components, and for me, my room, my ears, these have been a very good thing. 

Your experience may differ, of course...

David

Mullards are the quintessential warm tube for 12AU7, 12AX7, EL34, etc. In general, Mullards are warmer than Brimars. Unlikely you’ll find warmer tubes than Mullards. No doubt AI ripped off a plethora of posts recommending them, for that reason. The 4003 "box plates" in particular, because it was produced in large quantities all the way into the 80s for military production. Upscale Audio in particular sold them for quite a while. But don’t sleep on the Mullard 12AU7 with standard ladder plates, though they may be more costly. RCA black plates (NOT the "clear tops") have been known as warm too. Sometimes RCAs are VERY good, though I've had some duds too. 

@steakster @buellrider97 @mulveling @dtorc Thank you for your recommendations. I have added RCA ‘50’s blackplates to my list. They happen to be available from steakster’s recommended seller. Buellrider97, thank you for the offer. 
I have the UK vender checking to see if the CV4003 Mullards were manufactured in Blackburn. Also, need to consider all the duty / tariff fees. Thanks 

Mullards as a Mil Spec in the models discussed, are the antithesis of a typical Amperex equivalent. That noticeable difference in the sonic trait does not make the Mullard a warm, rich Valve. 

The Amperex can in certain circuits be similar in the end sound produced to resemble cleaning a Blackboard with one's nails.

The Mullard is a much more desirable implement selected to clean the Blackboard.

Certain circuits with Copper used in Cap's and without metal films in use, might give up the Valves inclusion as being with added depth to the sonic.