Why Rumble One Side of Album

My KAB RF1 arrived today.  I recently bought a Boz Scaggs Greatest Hits Live record that made my woofers look like they were going to jump out of their enclosures.  With the RF1 there was NO woofer pumping whatsoever. 

The clipping indicators on my amps often flash (I think they are overly sensitive!) when I play vinyl.  I turned it up louder than I usually listen and they did not come on at all, so it appears to have lightened the load on my amps also. 

I only listened for a few minutes (work and all that) but didn't notice any degradation of sound.  I'll be able to do some extended listening later this week.

OP,

Is it always on the same side?

@big_greg , I have a very effective digital  rumble filter 80 dB/oct @ 18 Hz. I would estimate that 2/3 of the rumble is above 18 Hz and clearly audible on many discs. Pushing a rumble filter up higher destroys your bass especially if it is analog. Having a rumble filter is no excuse for the people cutting lacquers on poor lathes. 

Is it always on the same side?

No..not always and sometimes both, but many records are dead silent.

@mijostyn the RF1 is supposed to cut at 20hz. I haven't listened to anything bass heavy yet. First impressions are very positive. Bass seems present and tight, nothing noticeable affecting upper frequencies.