how old is old? sounds like you might have some problems with the woofers. If an amp clips it clips the entire signal not just the lows, you will usually notice when your tweeter and mid no longer work.
Is this the sound of clipping?
I recently switched to some old Signet 260s, hooking them up to my Arcam Alpha 3. The 260s are about 87db in efficiency, the Arcam puts out about 40 wpc. In the large room that I'm playing it in, I get a course buzz from the speakers at higher volumes. But the sound is only bad on low-frequency notes. The highs and mids don't deteriorate. I never had this sound with my more efficient Spendors that I had used previously. I always assumed that clipping meant an overall deterioration of the sound, not just on the low end. Is it clipping or something with the woofers? I know that I could solve the mystery with the substitution of a more powerful amp, but won't have an opportunity to do that for a little while.