Tweaks for a Musical Fidelity X10-D tube buffer?

You'd be MUCH better off selling the 7700, Sigma II, and the X10-D. Then buy a higher quality one box unit.

What dvd player can you recommend? I'm using the sony for video playback only as I'm also using a Cary303/100 cdp for audio.

I'd suggest the latest and greatest from Pioneer:
http://www.pioneerelectronics.com/Pioneer/CDA/HomeProducts/HomeProductDetails/0,1422,2092,00.html

If you sell all of the aforementioned gear (including the Cary), and if you shop carefully, this is doable.

I upgraded my sony 7700 to the sony DVP-NS999ES and it makes a big difference, particularly in the added bass and clarity in the surround channels. A big improvement! And you get an SACD player as well.

The MF X-PSU upgrades the wall wart
and provides power for up to 4 X-units.
These are avaialable on audiogon and ebay
from time to time. They usually go for
$150-$200.
I use it with my X10-D (and other X-units)
and I think it is worth the extra cost.

Would you be comparing the analog out of
the 7700 followed by the X10 to the output
of the SigmaII? In my opinion, the D/A
conversion is what's important here. I'd
expect the D/A capabilites of the SigmaII
outperform those of the 7700.

X10 will only smooth out crummy output from
crummy op-amps in crummy equipment.
While often cited an an antidote to harsh
digital sound from CD players, the X10 is
actually useful for spiffing up ANY analog out
which uses crummy opamps.

A more useful comparison might be to compare the
SigmaII analog output to the 7700 analog out directly.
Then compare again using the X10 after the analog
out of each unit.

Have fun!!!