Bob, it is not apples-to-apples to compare the low distortion products of op amps-- which nevertheless at higher frequencies add audible grain & hardness & loss of refinement in the treble and midrange-- to the much higher percentage distortion observed in bass drivers-- which is often perceived by the ear as euphonic warmth. Op amps in the analog domain can be good or bad (usually bad). My experience has been that even a good op amp chip is highly sensitive to power source. It's rare at even higher price points to find discrete voltage regulation or clean battery sources applied to solve the problem. Not to mention possible inductance/capacitance issues associated with long IC runs to/from the sub in a configuration that utilizes the sub's inboard HPF.
Since early audiophile subs like the Muse came on the scene in the mid-90s, reviewers have consistently observed the hardening of treble that can occur when adding a sub's inboard HPF in the chain to the main amp. There are no absolutes on these points. Doubtless some main speaker/main amp combos benefit more than others from an inline HPF that relieves them from handling bass frequencies. But there is no need to introduce a trade-off between treble & bass performance, if the main amp/speakers are well matched. Short of spending $1000+ on an outboard Marchand or equivalent xover, I would advise sub users to stick to bass augmentation mode. Whether this is best delivered via speaker-level inputs or line-level inputs to the sub is another debate.
