can Audio Research 210's use KT88 instead of 6550

Yes, they are interchangeable.

I think they are interchangeable. The KT88 seems a better choice. The vintage Genalex KT88 seems to me the best KT88!

Strongly recommended!

Kisuh

I've used KT 88's in a D-115 previously to very good effect, and I really preferred them (except I didn't have a second amp to do a true side-by-side A-B test), but ARC do not recommend using them for some reason. I'd like to understand why this is.

ARC voices their amps for the 6550.
The 6550 likes to see a lower impedance output transformer, around 3200-3500 ohms. The KT-88 likes to see a 5000 ohm plate-to-plate output transformer.
The KT-88 can seem a little bloated on the bottom and a little veiled in the midrange, compared to the 6550, with the 3200-3500 ohm transformer.
Yes, they are interchangeable but will sound best in different situations.
Check with Leonard at ARC.

thanks very much ;-)