Inherent Difference - 12AU7 and a 12AT7

Well here is the issue; I have a Hovland HP-100 preamp and initially they used a 12AU7 in position 3 of the line stage. (Position one and two are both 12AX7's). In subsequent issues of the 100 they went to a 12AT7 in position 3. I can't even remember now which I am currently using but I need to replace them and am debating which way to go.

I love the Hovland but the tubes always get noisy fairly fast; after a year or so... I guess that while I am replacing the AX7's I could buy one AU7 and one AT7 and see what difference I could detect. I am just wondering if one or the other would help cut down on the quick degradation to noisy status...

Some info on the gain characteristics of the "12" family of 9 pin miniature tubes:(http://thetubestore.com/gainfactor.html) I would suggest some of the better/quieter, and longer lasting, European varieties(NOS- Siemens, Teles, Valvo, Amperex, Mullard). I've listed them, in order, from most transparent, to warmer/tubier. YES- Each manufacturer has a characteristic sound, in MOST gain circuits, regardless of the actual iteration.

Just because the company switched from an au to an at doesn't mean that you can. I'd bet they changed something or determined that more gain was needed and made the modifications. As Hovland before you even consider it.
As far as lasting 1 year, these tubes should last 3000 to 5000 hours. Unless you are leaving it on 24/7, you should get many years out of these. Even if they are wearing out, they shouldn't get noisy so easily. Usually your dynamics will just flatten out?

I'm sure that it is okay to swap between the AU7 and AT7; Hovland confirmed that for me several years ago; when I asked them what the differences were they said 'try it'and let them know what I heard and liked...

I think that I will just buy one of each and find out for myself. I have used the NOS Mullards from Upscale Audio but again these seem to get noisy way to fast. I may contact the Tube Store and get theirs and try something different.

Have anyone tried the outrageously expensive EAT 12AX7's? I am tempted to try them once; wonder if Music Direct would absolutely stand behind them and take them back if I was unhappy with the performance?

And, wonder why EAT does not make an AX7 or AU7?