Subwoofer w/ most SPL's For Music Only System

Of the subs you mention, only the Velodyne HGS 15 will be of excellent musical quality (around 1% THD). The other two are more for movies, the Earthquake has a passive radiator and is almost certain to have 20% harmonic distortion at the lowest frequencies....although it will probably go very loud at LF.

The only concern I might have with the Velodyne is thermal compression => it uses an elaborate voice coil system to get extreme linear extension....this is fine for an occasional movie earthquake but it might heat up on music with continuous LF demands...(voice coil heat needs nearby mass to dissipate otherwise resistance increases)

Since thermal compression and dynamic behaviour is very speaker/designer dependent...you really should consider a sub from the same people that make your main speakers ( the match at various SPL levels is likely to be much better if you stick to the same manufacturer - it is no good having an amazing sub if your speakers mid range compresses at high volumes levels and vice-versa is ture too - mismatch will just lead to frustration to get it to sound right as you change SPL level).

how much detail is in deep bass a bass horn will let you hear this

In terms of power, the horn design is very efficient but you will lose some details from transients and the same can be said for ported designs or passive radiators that boost LF power through resonance. For low cost and high SPL output design you cannot beat a horn.

A subwoofer loses detail due to massive driver excursion

Large excursions in a sealed box are indeed a problem for low cost drivers (small magnets, long voice coils) with correspondingly low levels of linearity (Xmax). The bass output will quickly compress and harmonic distortion will increase dramatically at higher excursions. (although this design may work very well at low SPL levels, more accurate than a resonant design)