Because it just absorbs all the amp can give and barely move or break a sweat it seems. Zpart of this may be their efficiency rating of i believe 85 . they are just a modest sey of fluance xl7f towers. They are rated for 40 to 200 rms. Just running them straight of denon no bi amp or bi wire. They sound like something is missing in the low mid range and almost like the highs are for a lack of a better comparison.....muffled or restrained somewhat. Hope that makes sense. I am running all fluance xl7f speakers in a 7.1 system all at correct height and sapced and toed in a bit to the listening position. As for the 1300 rating, it appears the 1200 is quite a bit more powerful as my manuel says 80 rms. Its not the spl im after so much as the clarity and ability to dig deep when needed.
Passive bi-amp and bi-wire won't do anything for your speakers.
The Denon 1200 should be the same amp as the 1300.
The speaker engineers recommend 40 - 200 Watts, so it seems the Denon is somewhere in the middle there.
Is the problem when listening to 2.1 music or only when watching 7.1 movies?
What is your Denon setting?
Audyssey Flat? Audyssey Reference? Audyssey Bypass L/R? Dynamic EQ on or off? Dynamic Volume on or off? What's your speaker, bass, crossover setting?
The midrange and treble do not require as much power as the bass. And if you set your speakers to SMALL, Crossover to 80-100Hz, most of the power requirement will go to the subwoofer, not the midrange and treble.
I can tell you right now that if the sound quality isn't as good as you want, it's usually NOT the amplifier section.
Unless the amp is
clipping (distorting noise/shutting down), the amp usually isn't the problem.
A few more watts or a lot more watts won't magically improve the clarity of the midrange.
The problem is usually the Processor/AVR settings, the speakers, or the room acoustics (hard floors, speaker placement).