Cary 300SEI - tube rolling/modifications

Hi all,
I am interested to know other people's experiences with tube rolling in this integrated. I am currently using a Sylvania 6sn7 military spec tube (preamp driver) along with KR 300b balloon tubes. I wondered if changing the two other 6sn7 tubes will affect sound at all...seems likely not but thought to ask.
Also anyone have any suggestions for modes to the amp other than standard Cary options? I really like the amp, that it has remote and also a headphone jack...seems to mate well to my Blumenhofer FS3 speakers...but would like to get some further subtle improvements if possible.I have heard some people use tubes other than the 6sn7 in this unit. Thanks for any help.
I hope someone can answer if there are some serious mods to be done to upgrade this amp further.....besides those outlined by Cary.

If I remember correctly, Kevin Deal and his crew at Upscale Audio used to specialize in Cary mods. Give them a call to see what they recommend
YES- Changing the amp's driver tubes will have a major impact,
on it's presentation. Bottom-gettered Sylvanias(VT-231/JAN-
CHS) & Ken-Rads(VT-231/JAN-CKR), from the 40's, both offer
nice sound stage, depth and clarity. At least as much
noticeable difference as changing the input tube. The effect
is cumulative. Try to get a NOS pair that are matched, triode
to triode, as well as to one another.

Thanks...I got a Sylvania Vt231 and just was able to get National Electronics Nu6sn7 black glass....just burning it in now. Those both made a difference in sound....and of course the KR Balloons are finally settling in...many wonderful hours of listening...thank you