Equalizer for McIntosh preamp - advice, please

I've used a McIntosh, the 104, 107 and 108. While this unit was good, it had a few short comings and only one advantage (it's relatively cheap). It's noisy, it definitely adds noise into the signal. I was bi-amping so the noise wasn't a big problem (because I used the device on the bass signal only), but I could not use the device full range as it really degraded the transparency in the midrange and treble. The other problem, as the limitation of frequency settings and Q factors (they only have 2 Q factors--which they call a broad band and narrow band--so I don't know what the actual Q is). It was actually the basic idea of getting the bass right through my personal use of the McIntosh that drove me to design the PARC, but without the above limitations. You can still get used ones from Audio Classics I believe.

You will see I'm very biased here, but our company designed a PARC, parametric adaptive room compensation system specifically for the purpose that you mention (room modes that have prominent bass bumps). It's much less expensive than most other eqs out there and sounds better (we've tested it in several very high end systems--the results were fairly amazing even in good quality rooms with Genesis 201 speakers). The PARC is on our website at:
http://www.rivesaudio.com/PARC.html. Also feel free to call or e-mail us with any questions you might have.

Clueless--maybe one of the few times I've disagreed (but to a small degree). Here's the problem. You are right that it is a time delay thing--but it occurs because of too much energy reinforced in the room. We look at these with waterfall plots and evaluate it in a psycho acoustical approach. That approach takes into account the time delay (it's actually an integral function and works similarly to the human ear). This also coincides with the fact that we can not perceive phase shifts at low frequencies. I know that there are many papers written that say we can, but our tests as well as many others show that the human ear really can not determine these things at low frequency. So all you need to do is reduce the total energy at that frequency (but it must be nearly exactly tailored for the Q factor and amplitude). Room treatment does nothing for the null points, they still exist. You should not sit in a null point, nor you should you sit in a peak. This is why 1/3 or 1/5 spacing is generally used--you don't have that problem in general. 1/2 and 1/4 are the worst places to be (but we always sit in the middle of the room from left to right don't we). This is true no matter what room treatment is used (we are talking about low frequencies only) or equalization. What happens is there is too much energy that is re-enforced by the parallel walls. You need to decrease that to bring things back into balance. Yes, the nodes and peaks still exist, but are far less problematic as they are in balance with the other frequencies (which also have peaks and nulls in the room). What's really interesting is to look at the energy distribution as a result of all modes (tangential and oblique as well as axial). A lot of surprises happen. CARA lets you do this to some degree and it's pretty fun to take a long look at what happens.
As to boosting frequencies where there are nulls--bad idea. The nulls still exist. If they are caused by impedence mis match between amplifier and speaker--it's a worse idea and will likely drive the amplifier into overload. Fortunately, particularly at low frequencies, the human ear is very forgiving to small (meaning narrow band) drop outs (or troughs) in the frequency spectrum.

I meant to comment in Ljgj's point out some of the pro gear he mentioned. The pro audio world has forgotten more about parametric EQ than most people will ever know. It's for this reason the PARC was designed by the engineers that make the Paragon, Legacy, and API mixing consoles. You may not have heard of these (unless you are into pro audio gear), but they are the absolute state of the art consoles, used my NBC studios and a number of very famous recording artists (the ones that can afford it and don't want digital). The boards are generally in the 250k range and up. And I can say working with this caliber of engineers has been an absolute pleasure. Their depth of knowledge in this area is remarkable.

Actually, Clueless, now that I re-read what you posted the quote is acurate--I would just add ...whenever possible.
The problem is that physical room treatment doesn't work very well below 150 Hz. Our unit works from 350 Hz on down to 16 hz, but we recommend that it only be used from 200 Hz on down. Above 200 you should really use physical room treatment.

Just to clarify. If you have a bass bump below 150 Hz, it's very difficult (not impossible) to treat it with physical devices. There are really only 2 physical devices for this: absorbers and resonators. An abosber that is efficient at that frequency (say 100 Hz for example) is only as efficient as the area of the space that is accentuating the frequency. So, if 2 parrallel walls are accentuating the frequency and you have an absorver that is 95% efficient at that frequency, but you only cover 5% of the wall space--then you've only made a 4.5% difference on the problem (not even audible). This is why people wonder, when they buy bass traps that are proported as being 90% or greater efficient at their frequency problem--why don't they hear a difference (or very much of one). The other way of dealing with problem is resonators. Yes, these can go to just about any frequency and almost any Q factor. The only problem is expense and size of their construction. They also have to be placed at very critical points in the room, where the peaks are maximized (not the pressure points like an absorber). In addition they re-radiate energy--so the design is critical in that the re-radiated energy doesn't cause more problems than the resonator solves. And yes, it's just under 2.5 octaves, but you can't use the piano as an ear test for this because so much of the energy is higher ordered harmonics. To test this theory you have to use pure tone generators don't have (other than what the room produces) harmonics.