Let's take this question in two parts. The first is whether one is able to get the maximum output power from the amp with this connection. The way to determine this is to find the power amp's input sensitivity spec and the maximum output voltage from the AVR's sub out. The input sensitivity of the power amp is the RMS voltage of a sine wave at the amp's input required to drive it to full power. The maximum RMS output voltage of the AVR's sub out must be greater than the input sensitivity of the amp (also specified in Volts RMS). There is a good discussion of input sensitivity in the thread, "Understanding Input Sensitivity specs
". Finding the maximum RMS output voltage of the AVR can be confusing. Sometimes they give a nebulous "rated output voltage", which can be, but is not necessarily just the RMS output voltage at which they spec the distortion, but not necessarily the highest output voltage it can provide with inaudible distortion. So you have to look carefully. Most AVRs can put out at least 2 Volts RMS, which is enough to drive almost any pro amp into clipping if its gain is turned up all the way. Some users are reporting 8 Volts RMS measured max output from their AVR. That's waaaay more than any pro amp needs.
The second part is a bit tricky, and is a subject about which there is a lot of misinformation. There's a great series of articles in EDN written by ace class D amp designer Bruno Putzeys about grounding and mixing balanced and unbalanced systems. It's not complete yet, but the latest installment was just published 06/10/2014.
Edit: I've added a link to the fourth and final part below.
The G word: How to get your audio off the ground (Part 1)
The G word: How to get your audio off the ground (Part 2)
The G word: How to get your audio off the ground (Part 3)
The G word: How to get your audio off the ground (Part 4)
In part 1, have a look at figures 5 and 6 on the second page. Figure 5 shows a balanced preamp driving a balanced amp. Figure 6 shows an unbalanced preamp driving a balanced amp. Note the caption of figure 6, "Symmetry is Useless". The "symmetry" refers to the fully balanced situation of figure 5, which he asserts to be unnecessary. Figure 6 is the setup I'd recommend for connecting an unbalanced AVR output to a balanced power amp input. Use a cable having two signal conductors plus shield, and an adapter at the AVR end to convert to a two-conductor configuration. One such adapter can be found here
, and would be used with a TRS (not TS!) cable. I have not tried these, so I cannot vouch for their quality. Monoprice sells their own version too, but in the reviews, lots of people are complaining about them falling apart. A less risky but more expensive and bulky alternative is to use an XLR cable with an XLR-to-RCA adapter. Neutrik makes these. They would have to be the type for which pin 1 and pin 3 are shorted internally.