My guess is that it will be borderline for symphonies, but fine for mildly compressed jazz, or highly compressed rock/pop stuff. 6C33 makes a lot of current so mild impedance dips to 3ohms should be OK if the transformers are good. If it is a cheapo Ebay type amp usually the transformers are the first thing compromised making speaker matching harder.
My 24W sepp 300B amps were loud enough on compressed music for the 90dB Ushers. But on less compressed stuff the dynamic peaks were weak, so I had to crank in more feedback for that. Those amps are not strong enough for Shane's Harbeth 40.1, which is an easy load but low sensitivity.