You inquired as to a speaker suitable for your Fisher amp. I looked at schematic on HFE, and it shows 7591 output tubes. This means that it is in the 25-30 watt per channel category. Might be more, but I am basing this guess on my memory of specs in RCA Receiving Tube Manual. The point is that this amp is not going to want to drive power hungry speakers like AR3's. This also suggests that your Cerwin-Vegas, which generally have a reputation for efficiency, would be a good choice for this amp.
Others have said that you necessarily want a ruler flat impedance curve, but I am not as demanding in that regard, because the output transformer will protect the output tubes. Tube amps are less able to produce the huge amounts of current demanded by inefficient speakers. My experience with tube amps is that they are less likely to be upset by impedance variations than transistor amps. I think you can use any decent sounding efficient speaker, and the more efficient, like the Cerwins, the better. I would look for a speaker rated at least 90 dB 1 watt 1 meter, and higher if you can get it.
A lot of guys invest in retubing these things and recapping them, which is a good idea if you are planning to use them in severe service. If you are not having trouble, amp may be OK, though. Turn out lights and watch the tubes, particularly the 7591 output tubes. If plates glow red at all, you need to stop using this amp and get it worked on. You will see the red glow on outside of plate and it will be readily visible if there. If the tube has any visible reddish coloration at all, you have a problem. Even a slight reddish quality is too much. I am not talking about filament glow, which comes from deep inside the tube and is yellow. Also you should put tubes in a tube checker and be sure that they are not gassy or grid-emitting or otherwies debilitated. A tube checker will spot these issues.
I believe that this unit has a signal processor loop. You can put the Soundcraftsmen 2217 unit in this loop and use it as an additional tone control. However, you should be able to remove the processor completely from the circuit so that it is only in operation if you are directly using it. (If you want, you can just use a DPDT switch to switch around the Soundcraftsmen.) You can also put the Soundcraftsmen on tape loop. That will allow it to be used only when tape monitor is in. I have a Dyna PAS-3X with a little box that provides old EQ curves. I put it on the "tape 2" input of my Hafler preamp, and I only switch it in when I am playing 78's. Adding in extra circuit elements will tend to add additional noise and distortion.
Other speakers? EV 12TRX-B's? Altecs? I remember the local HIFI store when I was in college, where they had tube amps similar to this one, and drove a set of Altec A-7's with them.
COOL! LOUD! And well within the capabilities of this amp.
And good luck from the old efficient old tube checker,