Dear Velocityyofhue: As I posted and from my point of view and experiences the main target to add subs to your audio system ( almost any system ) is not to go deep in the bass or higher bass quantity but to improve the quality performance of your Thiels ( and achieve a lot whole better system performance. ) lowering its high IMD/THD ( intermodulation distortion/total harmonic distortion. ), this target when you achieve it makes a difference like night and day, period.
Now, your Thiel's are a very good example on the subs subject because the Thiel woofer handle a wide ( more than five octaves ) frequency range: 28-30Hz to 1Khz ( due to its first order slope crossover. ).
That woofer needs to reproduce at the same time a 40Hz frequency and a 500Hz one ( example, the reality is more complicated due that the woofer has to handle all the frequencies in its very wide range. ) where the needs to the 40Hz frequency reproduction stress ( IMD ) heavy the 500Hz frequency reproduction where this one loose its un-distorted/clean reproduction that we heard like a smeared non-transparent no precise definition, etc, etc.
I know that you like what you are hearing, well you can't imagine how good is your system till you make that the IMD in your speakers goes really lower and one of the alternatives is through the integration in true stereo fashion of two self powered subs blended seamless.
When you achieve this the Thiel's woofer will work between around 80 Hz and up lowering the IMD that clean-up all the frequency spectrum response, now that woofer will work with out the " stress " ( excursion, overhang and the like that affect seriously the whole woofer quality performance: a heavy compromise. ) in favor of overall speaker quality performance.
At the same time the whole bass response will improve its quality performance because right now that bass frequency range is handle for a dedicated ( was expressely designed for ) speaker ( the subs. ) and for a dedicated amplifiers that was expressely designed to match exactly the needs of that subs drivers.
Here you will have and wll hear the quality on the bass that you never heard in any full range speakers ( non powered ) at any price.
You not only improve the quality of the bass response but obviously you improve to in the bass quantity and in a lower/deep bass response that you will hear only when the recording demand it.
Everything change for the better: power handling because your Krell will be working in a less wider frequency range: its overall performance improve, sounstage will be a lot better too, there are so many improvements that is almost impossible to name it.
Other thing: right now the position of your speakers is a compromise between the in room bass response and the mid/high response, there is a trade-off.
When the " deep " bass liberate the Thiels then you can move it/change a little its room position to achieve a better in room mid/high frequency range response/perception and the subs will be locate too at its best in room position: best of two " worlds "!
IMHO it is worth to try it, the change is of paramount importance with several and true rewards.
It is almost useful/vain to argument against the subs with out listen/try the whole alternative.
Of course that some speaker manufacturers and/or audio dealers/reviewers can't support the subs " road " because the alternative goes against business ( $$$$ ) where the money is on sales of bigger ( more costly ) ( next model in a speaker line. ) or different speakers ( more expensive )than in two inexpensive subs that we even can buy second hand.
But even IMHO if we are wealthy enough the fact is that we can't achieve better quality performance ( everything the same or that the main speakers were decent speakers. ) with the new " big " speakers " than with the un-expensive subs alternative.
Regards and enjoy the music.
Raul.
