What tube amp to drive Guarneri's?

What do you mean "add" a tube amp?? Play the two amps in parallel?? Or one tube amp for full-range?

If the latter, and having tried a few -- but obviously not all -- tubes the only model that sounded OK was a KR sporting one big 611 tube/channel. Dunno what it;s called, but it's distinctive.

Why not stick to your Rowland?

I remembered the KR enterprises amp model: It's a KR-"Kronzilla" using of course a 1610 tube (not 1611).

It's a SE with ~25W/ channel. They also have monos (~40W/channel SE) which the Guarneri owner ultimately ended up with.

Since you asked -- albeit in a different context -- the room here was big (~600 / 60m2)