I guess that I am going to join the andti-Bose 901 hordes. The issue with the Bose 901 is that it shoots most of the sound against the back wall, meaning that you will get what is basically mush, rather than any kind of image. If I were in your shoes I would get more conventionally designed speakers. But before you try anything else, take your Dynaco A-25's and move them into the larger room and play them spaced more widely apart and with tweeters about head high when you are sitting down, or about 2 1/2 to 3 feet off the floor. I think you may find that you like them that way, and that they will give a "more diffuse" sound set up this way, so long as they are a couple of feet from walls. You can build or improvise stands from 3/4 inch plywood and a couple of 2 x 4's. You may decide on other speakers that will work well, depending on your taste.
The real problem with the 901's is the same as the problems with a lot of Bose gear, which is too many half-solved engineering problems. The 901's don't work right without the special Bose equalizer to bring up the bass and the high end, and this will tend to increase distortion in bass and high end from your amp, because you are wasting power driving the equalizer, rather than using it to drive the speakers. I heard Bose 901's many years ago, when they were new, and I didn't like them even then. They were mushy and lumpy.
I recently inherited a set of Infinity P362's and a set of Infinity RS76 speakers, and I think either one would be suitable for a larger room, with the RS76's giving a better low end for most popular music. P362's cut off about 40 Hz. I normally build my own speakers, but I did experiment with these speakers just to see what they sounded like, and they were OK. Of course, they may not be to your taste.
And that, of course, is FWIW. My opinons and a buck will get you a cup of senior coffee at McDonalds. And good luck from the old coffee drinker,