The VAC Renaissance 70/70 was lovely on my old SC-III's, having more than enough power and all the finesse you could want. Extremely well-built and simple to use, too, because the tubes are self-biasing and the amp has a tube failure circuit that shuts down any tube that starts to die. As for whether it will give you the power and headroom of a 300 watt solid-state amp, perhaps yes -- the amp is totally dual mono, all the way down to separate power cords and on/off switches, and has very high quality output transformers and power supplies -- it sounds much more powerful than its wattage rating. It replaced a Bryston 4B-ST (290 watts/channel) in my system with no perceptable loss in horsepower and big improvements everywhere else. The 70/70's are now going for +/- $4,500-$6,000 used (they are still in production and list for $14,000). It uses 300-B output tubes, which last many thousands of hours longer than pentodes (6550's, KT-88's, EL-34's). I run the monoblock version of the amps, the Renaissance 140/140's, in my main system.
I can recommend three other amps.
The CAT JL-2 stereo amp is transparent, sophisticated and enormously powerful (it uses a 55 lb. output transformer), making most 300 watt solid-state amps sound neutered. Incredibly well-built (weighs 177 lbs.). $12k new and $6-$8k used.
Lamm's 90 watt/channel monoblocks are powerful, transparent and extremely well-built. $22k new and +/-$9k-$12k used.
The Tenor 75 watt monos are great, but the company is evidently out of business.
All of these amps will challenge your Dunlavys.
Good luck.