I just created some -20dB test tones via Sound Forge at the frequencies I wanted - 125, 100, 80, 63, 50, 40, 31.5, 25 & 20hz - so that I had a constant signal to measure against. All of my test CDs etc have sweeps so it is impossible to get accurate readings.

Dave

M60s, VP150, QS8s(3), SVS PB12-isd, Denon 3805, Emotiva UPA-7