This might sound odd, but try a dummy load and see if their is a difference.
You might be surprized. Does the output peek and current dip match? if you
are really worried, call me and I'll volunteer to take the radio off your
hands. Put 6146 W's in it and let it roar. By the way, are you sure those
tubes aren't beefed up versions of the 6146 already?
w's would max out at 300 watts in and 225 out. I have seen 80 per cent
effeciency on a great radio and set of tubes. That would be 240 watts out.
If you have b's or w's relax, enjoy, and blast away.