At reference, what should a subs response curve look like?
Just trying to calculate or see if I am hitting the DB required.
On REW at 0db, I hit 85db running the sub cal frequency outside of right or left speaker which I know isi 10db hot.
I am flat 10hz to 80 pretty much.
But what db should each frequency be hitting? Where should I be hitting at say 20hz if I'm at reference on a response curve if I'm running flat 0db? Should it be 75?
You have actually asked a couple of different questions mixed together. The first question is: What are Reference volumes? And, the second question is: What is the lowest frequency where I want to be able to hit Reference volumes?
The first question is relatively simple. Dolby/THX Reference for 5.1 movie content is a nominal average volume of 85dB for the regular channels, with peak volumes of up to 105dB for the regular channels, and peak volumes of 115dB for the LFE (low-frequency effects) channel. The LFE channel is intended to be played by subwoofer. It is recorded +10dB louder than the regular channels, which are played by both the regular speakers and the subwoofers, with crossovers determining where the subs take over from the speakers.
People sometimes get confused by the test tones that our AVR's use when they calibrate our HT systems to Reference. The test tones are only 75dB, instead of the 85dB that we might expect them to be, because people found 85dB test tones too loud. So, the test tones were reduced to 75dB. But, our AVR's are programmed to automatically add another +10dB to those 75dB levels, so that at 0.0 MV (master volume) our regular channels will play 85dB with the potential peaks mentioned above. (They are only potential peaks because not every movie will have volume peaks that loud.)
Just because an HT system is calibrated to Reference though, doesn't mean that our speakers and subwoofers can actually play those volume levels, especially without distortion. That depends on the inherent capability of our transducers and their distance from our MLP (main listening position). Calibrating our systems to Reference simply gives us all a common starting point. (As a side note, 0.0 MV on a calibrated system is quite loud for most of us.)
As for the second question: whether your subwoofer(s) should be able to hit maximum volume levels of 115dB (peak LFE volumes) at 20Hz, that is a question which depends entirely on your own listening preferences and the inherent capabilities of your subwoofer(s). If you want to understand some of this better, both Dolby/THX Reference and what occurs during an Audyssey calibration, are explained in a great deal more detail in Section II of the Guide linked in my signature.