Attenuators definitely "color" (to use nice language) the sound of an amp, I'm not a big fan of them.
Also, that 300T has a
whole bunch of energy to try to absorb, if you find an attenuator that will handle it you could probably also grill hamburgers on it.
Have you thought of using a Variac? You could probably get the effect you are looking for that way.
Reeves Amplification (
www.reevesamps.com) uses power scaling licenced from London Power, quite slick and it's built in to the chassis.
I'm very familiar with Reeves gear, top notch quality and the power scaling does what they say it does; takes a 50 watt amp down to 1/2 watt, but with
full power tube saturation, unlike a master volume that only saturates the preamp tube(s).
Pretty amazing technology but I don't think it will handle 300 watts.
So much for the endorsement...
I'd go with the Variac.