This topic seems to come up in different threads from time to time. It seems timely to me right now, because I'm experimenting with a similar thing. I have several amps -- some head/cab and some combo. I play 6-sting, so I cover a wide frequency. It would be nice to have a big fat boomy bottom and also enough punch and definition to insert high-end fills or chord jabs that don't get swallowed up. Doing this with one amp is difficult, because the sounds interfere, either in the amp or in the speaker cab. Lately, I used a Boss DD-7, which has a mode allowing a mono input to be split into two outputs. In my case, I add a small amount of delay to one of the outputs while the other is completely dry. I thought the delay might intensify the separation between amps, but it really doesn't produce much useful result.
In my practice space, I'm running the delayed signal to a Fender 100 combo with 15 that has never had much high end. I'm turning bass control up and cutting the treble. It sounds mushy and boomy and great on the low-pitched strings. The dry signal goes to a Hartke HD75 10 with flat EQ, except a bass is about -3dB. It has nice definition when I play double stops or snap the high strings. This seems to do what I want to do. I'm thinking of moving the concept to a pair of bigger amps for a gig (whenever that might happen).
I agree with commenters who say that balancing is a problem. (Seems like the major issue to me.)
Also, some people seem to think that signal phasing could be a problem. Electrical signal phasing and phase interference of acoustic waves in a 3D space are two entirely different things. Acoustic phasing is unlikely to lead to any practical issue. No one ever says "I will NOT allow the FOH to reinforce the sound from my onstage amp, because phase differences might kill my sound." That's largely because audible wavelengths are small compared to the size of most rooms, and because the wavelength is different for every individual frequency.
Sorry to be blunt, but much of what you are saying is just flat out wrong. Often when people are talking about phase they actually mean polarity. So when they say a cab is out of phase, they mean it is out of polarity.
When a signal is out of polarity the sign wave is flipped 180 degrees.
If you were to overlay these two signals they would cancel out.
If you summed them electronically. They cancellation would be almost perfect. If you summed the signals acoustically the cancellation is less perfect because the sound is not coming from the same location.
Depending upon the distance between the sound sources, and where you are standing, the cancellation will be different. Different terms such as lobing and comb filtering are used to describe what happens. But they are really just different ways of describing the same destructive interference patterns. If you are curious, please take a look at the attached "bass array guide."
Another thing, phase differences can be small or extremely large. For example you may have a phase difference of a few degrees or of several wavelengths. The phase differences can be time based, in other words one sound source is delayed with respect to the other, or physically displaced with the respect to the other so sound from two sound sources arrives at the listener at two different times.
There are also instantaneous phase variances. For example with crossovers you have two filters sections, a high pass filter (HPF) and a low pass filter (LPF). Within the effective range of the crossover there are instantaneous phase variances that are determined by the slope of the crossover filters. For example with a 48dB per octave filter you get 720 degrees of phase shift. To make it even more complicated, the phase leads in one of the filters and lags in the other filter, so you get 360 degrees of shift in the HPF and 360 degree of shift in the LPF. Ideally the filters are designed and aligned so their phase response is complementary or matches. If this does not occur it causes dispersion problems throughout the crossover region.
Also, loudspeakers do not have perfect phase response. If you were to look at the phase response of a typical speaker, as the frequency is moving from high to low, you will see that the phase constantly changes. The rate at which phase changes varies from one speaker design to the next. Here is an old TB thread if you are interested:
Mixing Cabinets and Phase Response.
I have not read it, and I don't claim it's a great thread, but the first post does have some decent graphics.
Here is the first phase chart from the thread:
Based on what you wrote in your post, my guess is you want a good quality extended range bass cab. Multi-amping is generally something people use when they want to run a clean amp and a distorted amp.
Biamping is something we used to do a long time ago to try and get more of a HiFi sound with extended range. But current amplifier and speaker designs are better than most people can do with biamping.
A lot of engineering goes into a modern speaker. When you assemble a biamp or triamp rig, you are most likely just cobbling together random components rather than selecting components that will really work well together. Also unless you really have the skills and tools to properly align and tune the rig, you are not really putting the necessary design into the system to make it perform at the level of a good modern full range bass rig. IMHO it's possible to still get superior performance out of an active bass rig, but it will expensive, heavy, large, and you need the necessary expertise.
Good luck on your journey!
