Large speakers plus EQ, what have you done?

You can "eq" the speakers through component selection in the crossover (even if unintentionally) and this is probably why some people prefer passive crossovers to active crossovers.

Case in point. All the values in the crossover have an "ideal" combination, but the combination can be tweaked to produce a hump or a dip in an area of the band that is pleasing to the listener.

The 5.8 mH choke with an 8 ohm load would actually like to "see" a 44 mfd cap to ground. A larger cap (e.g., 47 mfd) will create a greater roll-off, but it will also create a hump just below the crossover frequency. The larger the change from "ideal", the larger/broader the hump. So if you set the crossover to the "right" frequency and you "over-capacitor" it, you can add a "hump of bass" just below this frequency (which I have at 310 Hz).

Similarly, the combination of C2, C3, and C4 if "over-capacitored" will increase the roll-off, but create a hump below the crossover frequency, which puts it at ~1kHz where the ear is most sensitive.  This takes a bit of the high end hash out of the 2251 while giving voices a warmer, more pleasant tone. 

I've had a pair of Altec 604-8G in smaller than optimal cabinets, about 5 cu ft.    Spent a bit of time with REW and placement in a fairly large room, ~ 500 sq ft, and found the best overall support for the bass was, you guessed it, close to the side walls.  My Cardas speaker placement guide was laughing at me.  But the bass was strong down to 40 Hz with no EQ, and the nulls due to placement weren't noticeable.  But the horns seemed a bit peaky in the 1-3k range with certain vocals and brass.  Given the age of the stock crossovers, I made some using J. Markwart's simple crossover, listened to them for a while, but never quite liked the sound.  Went back to the stock crossovers and added some very steep notch filters at two of the frequencies that tamed most of the peaks.  Still short of perfection, but they're very pleasant to listen to for hours, and quite forgiving of less than ideal recordings.  To summarize, placement did the most good for bass, and getting control of the mid band peaks was the second step.  Have pondered DSP, convolution files in JRiver, etc.  It's all about the journey.....

Hi @toddalin - Every passive crossover is an EQ, but that’s really not what I wanted to discuss especially as most hobbyists aren’t going to mod anything about their passive crossovers.

While we can, generally, adjust the speaker to the room to match output with room gain what we can't really do efficiently is deal with narrow room modes.  I mean, it's possible, but then you end up with a very custom speaker that only works in one place with probably a large extra outlay in parts. 

pickindoug, these eq units are what you need, but they are probably "one-offs" made especially for my "one-off" tri-amped Super Big Reds with their "one-off" electronic crossover. I bought these from a recording studio that went under.

Yamaha NS-5000. 12 inch woofers. 26 hz. Very satisfied