Yes. I'm switching to stereo mode while listening to music. I also use two subwoofers to listen to music and movies. Bass is all adequate and I'm very much content for both movies and music. I can adjust the crossover setting manually at any time. I can use the EQ if I turn the Audyssey MultEQXT set to off.
I know Denon/Marantz would set your monitor 9 or 11 to 40 Hz because they do go that low in your room, but I have plotted too many graphs to know typically 80 Hz will give you
much smoother bass response from around 20 Hz and up. You may, or may not as it depends on your room to a large extent, feel it sounds thinner as the response gets more accurate that is flatter and smoother. That's because with 40 Hz XO, you may just happen to have some major bass bumps that got leveled off with the higher XO. It is worth a try, and I can almost guarantee if you like smoother and tighter bass instead of the exaggerated bass then you will enjoy the music more. Also, do avoid dynamic volume if you can, but do turn on DEQ because it will compensate the perceived loss of bass at volume below 0 in a progressive manner. With it off, if you feel the bass is good, it is really due to the bass bumps created by the interaction between the Monitor 9, PB1000s and your room.
If you don't listen loud, indicated by your volume being at -38 to -10, an external amp will do nothing for you. Regarding the monitor 11, I did check the specs, and would say your dealer either don't know much about the power requirement as it relates to speaker specs, room dimensions, seating distance etc., or he/she would like you to spend money on an amp. Power not used is not going to make any difference! If you SR5009 has been calibrated properly, it should give you cinema loud level at your seat at volume 0, and still have at least 3 dB of headroom, and that's just with 1 speaker!!
If you do listen at or near volume 0, then an amp should help but then the 7025 is not worth it as it may get you 1 to 1.5 dB spl at the most.
You will be much better off spending $899 on a Denon AVR-X4400H, or $599 on a Denon AVR-X3400H, either one has a much better DAC then the SR5009 and most importantly, Audyssey XT32 Sub EQ HT that can dial in your speakers and two subwoofers much more accurately. Those are fantastic deals right now because the newer models are out. You can also go with a SR6012 for about the same price as the X4400H but the X4400H is a better deal as it is basically a SR7012, without the proprietary HDAM (with no audible benefits in my experience) modules and the useless 7.1 analog inputs.