Hi. I have an SPL Goldmike 9844 as one of my main pres and am wondering would changing/upgrading the tubes improve the audio quality? I use this pre mainly for vocals.
If changing the tubes would be an improvement which tubes should I go for? The factory tubes are ECC 83 dul triode.

ECC83 is a Euro designation for a 12ax7, and the US military designation is 7025. I would guess stock tubes are probably low-end Sovtek's or of Chinese manufacture. Lots of choices in current production, some of which are plenty good. Then there's NOS. Can't really tell what an improvement would be without knowing if something is wrong with the current sound.

Bear

I got a Millenia TD-1 that they put a Mullard tube in. Does it sound any better? I don't know. I like to think taht it does.

I do know that I've got years and years experience with tubes. If it says EI then it's probably not too bad. My experience would be that NOS Sylvania would be probably the most reasonably price high quality tube available. Otheriwise, if you're going new, then maybe Tung Sol. People who make quality tube gear throw out a good number of tubes to get the ones they ship with their boxes so please don't think the law of averages get suspended because you're just trying to improve your mic pre. You'll still encounter bad tubes.

One more thing... It's been my experience that when you go from the Euro designation to the US model number to the military model number that you'll get different amounts of headroom as well as distortion.