KT 91

I believe like the Gold Aero KT99 it is a KT90. I also assume they are made by Ei. If so the answer is yes. I used KT90/99 in place of 6550 and KT88's.
Do they look like any of these?
https://web.archive.org/web/20110426202420/http://www.manleylabs.com/galleria/KT90.html

The KT91's are AR branded Ei KT90's. IOW the KT90/91/99 are all the same tube. Those are some fine tubes IMO. Electro Harmonix has a KT90 along with Ming Da. Neither are even close to the Ei in terms of SQ.

That said If you are interested in Ei KT90's ideally you want the type2 or early type3. Type1 and later type3 were very unreliable. The Ei Kt90's are somewhat hard to find but since they also somewhat unknown they can be had for a good price for a vintage tube. A real sleeper of a tube. They remind me of an EL34 Midrange with much better bass and highs. Wider soundstage too.