"Greg Williams" wrote in message

Have you listened to the Adire Audio subs at all? I have been
looking at the Rava (based on the Shiva driver) and the Daeva (based
on the Tempest driver but temporarily unavailable).

I was thinking that a 15" sealed sub would be optimal, but I'm not
sure it would work with my monitors. I have to hi-pass them a little
high at 100Hz or else the midrange gets muddy. I'm not sure if a 15"
driver will integrate that well at that high of a frequency or not.

http://www.adireaudio.com/

Only half a question - not a clue what the monitor speakers actually are.

I can share these experiences, both of which I consider to be sucessful.

(1) NHT 2.5i with 8 inch woofer crossed over to an 18" subwoofer at 58 Hz
(2) NHT Pro A10 with 6.5" woofer crossed over to a 12" subwoofer at 130 Hz.

In both cases I effected the crossover with 24 dB/octave active crossovers.
The 2.5i system was based on a Rane MX22, while the A10 system was based on
a DBX 123.

IME, the real problem relating to crossing over speakers at high frequencies
is a matter of bass imaging. Bass is definitely directional at 130 Hz,
which means that the subwoofer needs to be acoustically centered and
relatively close to the upper range speakers.