I'm pretty sure it's just an amplifier. I took a very quick look at
the schematic this morning for my HP8640M, which should be the same.
I believe it's 18dB gain. As you say, it must be capable of the
output power. GALI-51 would be a bit closer match for the gain, but
not quite enough output power?

Cheers,
Tom

Parallel a couple.

W4ZCB