Subwoofer for classical music listener

Two ideas regarding subs that I consider very true and important are:

1- You shouldn’t hear a sub. A good subwoofer placed optimally in a room should just make the loudspeaker it is augmenting sound as if it now has response to a lower frequency. It should be invisible.

2- It is often the eigenmodes of the room you are hearing when adding a sub to a loudspeaker, the lower frequencies now reproduced exciting the rooms resonant modes, rather than the sub itself. "Room boom" is often incorrectly attributed to the sub(s).

The best way to minimize both the above is to employ dipole subs, rather than monopoles. A dipole sub excites fewer room modes (no sidewall-to-sidewall, dipoles having a null to either side), leading to fewer standing waves. The Magneplanar Tympani bass panel is a great, musical sub, as is the GR Research/Rythmik OB/Dipole Direct Servo-Feedback Sub.