Ahhhhh too many questions...........

First off, if it was me I wouldn't add another mic, I might even remove
one of the ones you are using now.

What are you using for a pre amp now?
A good pre amp will help alot. Just watch out for the not so good pre
amps and the starved tube, so called tube mic pres.

Yes you can get a good mic for under $1000, even under $800. It
probably won't have a tube in it though. To get a decent tube mic it's
gonna cost you over $1000 though, more like 2x's that minimum. Possibly
if you have to have a tube you could dig up something like a AKG C61,
even a Neumann/gefell M582 will only run you about $700, and the M94
capsule would be good on the acoustic guitar.

If it was me doing it, first off I'd get a good tube pre, before I got a
good tube mic.

The 2 ch Peavy is a real tube pre and runs about $600-700 used on ebay.
A Telefunken/tab/seimens V77 module would be better, but then you need
to add phantom power, and some way to attenuate it(like a shure variable
pad), and a step up transformer as well.

I say keep the SM81's, they aren't that bad. Used in X/Y your going to
get minimal if any phase cancelation. Now take your $1000 and get a
good, REAL tube pre amp. If you dont' know which ones are real or not,
ask here, I"M sure some one will recomend one in your price range(like
the Peavey)




Twist Turner
http://tinyurl.com/ul70