Wow. This is it!
I don’t get it quite to your level, but the dB increase quite a bit. At minus 3 it is approaching 90 which is more than enough. But then my sub amp gave up so I ordered a new resistor it seems to be a common problem.
Now, technically, what would happen if I added a third woofer to the speaker and just soldered it to the same place at the crossover (LF) as the two other woofers?
What is your listening distance? This is one of the most important thing to measure at your listening position before concluding your amp power requirements. If you sit at 1 m, you should get about 90-92 dB based on measured result of those large Klipsch speakers such as the RF-7 that the specs claim 100 dB but if you do a power search on the internet you will find that based reports by those who apparently measured it with pink noise or claimed according to Klipsch engineers (as opposed to marketing) the actual sensitivity could be between 93 to 100 dB /w/m.
On the conservative side, I would therefore consider the RF-7 can do at least 93 dB/w/m but only 90 dB/2.83V/m because they surely look more like 4 ohm nominal that 8 ohm nominal.
Now, if you calculate the "power" required, or use an online calculator you will get the following:
1) 81 dB from 13 ft, with one speaker placed near a wall
2) You mentioned C40, volume at -8 to -5, lets use -6 to make the math simpler, then if at 0, post Audyssey, you should be getting 85 dB at your MLP, so at -6 you should get about 102 dB.
3) The 102 dB I arrived at in 2) is not consistent with you posted
"I took a measurement from listening position now, an SPL there is high in the 70’s as far as dB. It can do maybe 83 but this is as high as I want to push my setup. this is about minus 5dB on the amp volume, and the amp heats up. "
So I am interested to know which spl meter you used, often people used free phone apps that aren't very good for such purposes. I can see a discrepancy of a few dB, but the 102 dB (
@dlaloum 's number's show even a higher number because he had an error in the distance factor), and you measure 83 dB is too much and that's what made you wonder why you would need to add that XTC amp right?
3) With the C40 volume at -6, the preamp output voltage will be slightly above 1 V, nowhere near it's clipping point of at least of around 4 V. So the C40 is not the source of your perceived distortion rising at volume -6, but at that point, it's build in power amp will be close to or exceed the clipping point.
4) It is important to understand that just because you are getting 79 dB (even if that is the right number) from your MLP at around volume -5, does not mean the preamp voltage will be just above 1 V, I would use 1.12 V for easier math, and the internal amp will be at the rated 125 W because:
a) The Marantz C40's internal amp's gain is about 29 dB, so if the preamp output is at say 1.12
V, the C40's speaker output will be at its rated 125 V, but wait, that's into 8 ohms, the RF-7 is more like a 4 ohm speaker so to the C40, it would have to deliver twice as much current, and the equivalent wattage (a wrong term to use but I have to go along with it for now) will then be 250 W.
b) More importantly, all such calculations, estimations above are based on standards that while should be applicable to bench tests and movie tracks, for music there is no such standard, we all know some CDs, stream sources such as Youtube, Qobuz, Amazon etc., do show variation in their recording level, so volume -5 may only push the C40 to output 250 W average, it could/may push it to well above that if you listening to various streaming sources and various discs such as CDs, SACDs etc.
Based on the above, if we trust science, for your seemingly very loud SPL requirements, no AVR can do it for you, not even the most powerful Denon, Yamaha, and Onkyo flagship (I mean the real flagships) from 10 plus years ago can do it for you. You did the right thing adding the XTC power amp.
Conclusion
Yes you do need an amp like the XTC, as you seems to be willing to listen to reference level or higher. I hope you don't do that for longer than a few minutes continuously at such high level.
Maths and science works, and thanks to the details you provided so far, except the distance, so I have to assume 4 meters but you can made adjustments easily as follow:
used sensitivity 90 dB/2.83V/m or 93 dB/2W/m, 2W because RF-7's nominal is likely closer to 4 ohms
Spl vs distance:
I rushed a bit, if anyone noticed errors below please alert me, thanks.
1 m.............................93 dB
1.5m...........................89.5 dB
2m..............................87 dB
2.5m...........................85 dB
3m..............................83.5
3.5m...........................82.1
4m..............................81 dB
That's for 1 W, so for 2, 4, 16, 32, 64, 128 W, double add 3, 6, 9, 12, 15, 18 dB accordingly, for example:
4m, 128 W will get you about 105 dB that would be reference level, so, 128 W output from the C40 will get you reference level at 4 m, but that is with it driving just one speaker
The C40 will not be able to drive such speakers on a continuous level at reference level and it will fail if you try too often. Again you have done the right right thing adding the XTC power amp.
You would be okay without the help of the power amp, if your distance is 2 meter, or you dial your volume down to -10 or lower.
