You seem to have a few misconceptions, or just misunderstand, so may be consider the following:
1) Yamaha's RX-A2080 should be capable of 2 V or higher. The specified 1 V is not the "maximum". This has been mentioned many times, that those pre out specs are based on one specific condition that manufacturers rarely detailed in their published specs, so you either have seek out best test results, or purchase the service manual to see if you can figure out what the maximum output voltage is and at what distortions level.
2) Depending on your actual need, if you listen to well below THX reference level (your volume setting is a good indicator) like most people do, chance is good that you may have never reached even 150 W peak, or much lower. It is nice to have 1.5 V, or, obviously 2.0 V or higher would be better so there is room to grow.
3) You are correct that the AV7706 can output 2.4 V at around 75 dB SINAD from the XLR (note that's still way worse than any of the Denon AVRs (those with the same AKM DAC chip used in all of the Marantz models too before 2021) measured on ASR. Again, that's not the maximum, the maximum would be more like 8 V, or 4 V from the RCAs.
4) Even if you use the 7706's XLR output, you
will not gain anything if you use the MM7055, except of course XLR would allow you to use much longer interconnects. Yes, you will get 2X the voltage of the RCA outputs, but the MM7005's gain will be 6 dB lower if you use the XLR inputs so the net results would be the same. Many power amps such as all of Marantz do that, so whether you use RCA or XLR outputs of a matching preamp, the output to the speakers will be the same.
5) Because of 4), you need to read the specs of the power amp of your choice carefully, and beware that that such specs could be confusing, so your best bet is contact the manufacturers/customer support. Examples of the potentially confusing specs vs reality that I know of include ATI, Outlaw, and probably Parasound's too. Or if you have the amp already, just measure it to see the difference for yourself.
Regarding your "additional" question about the SR8015, yes you can make the SR8015 to output 1.5 V by apply the required input signal level. For a beginner, an easy way to do it could be just use the AVR's generated test tone for signal, hook up a good multimeter to one channel of the pre out, say the center channel, crank the volume up gradually and watch the pre out voltage increase. As a quick and dirty test, I have done that to my Marantz AV8801 and Denon AVRs and was able to push them to output well over 4 V from the RCA outputs. When I did that stupid test, I disconnected the speakers because even with volume at -10 would be much too loud to bear. At that point, the output voltage would have been around 0.6 V or a little less.
When watching movies, if your volume is set to say, -10, the pre out voltage of your preamp would not likely exceed 0.6 V during peaks, averaging much lower, probably less than 0.1 V and obviously would vary with the movie/music's content. The pre out voltage always depend on/vary with the input signal level, and that in turn, depends on the media contents and their recording level.
There don't seem to be any reviewed bench measurements for the 2080, but there is one on the 1080 and it's pre out was measured at up to 2 V (it doesn't mean it won't go much higher, but Amir's standard tests on the RCA output typically stops at 2 V):
This is a review and detailed measurements of Yamaha RX-A1080 Home Theater Audio/Video Receiver (AVR). I purchased this solely for the purpose of testing through my company (Madrona Digital is a dealer for Yamaha among many other brands). The RX-A1080 is part of the upper tier of Yamaha AVRs...
www.audiosciencereview.com
