When I play both speaker paris separately, the B&Ws are clearly better. The Technics sound narrow and a bit muddy by comparison. When I play all four paris at once it sounds incredible. The Technics have something in the mid-bass range that the B&Ws lack. When you play all four at once and then cut the Technics, it leaves behind a giant hole in the audio.
I've tested this on a lot of different styles of music like rock and electronic. It's not a big issue there. It's really glaring on acoustic and classical music. Specifically the lower harmonics of string instruments like guitars and cellos
Is this just how the B&W 683s are? Is there something I can do to fix the issue?
First of all I can't be certain you have anything to fix. You have old Far Eastern speakers that were never good to start with, and now you have put in drivers, not designed for that box and more importantly the crossovers. So you can guarantee you have been listening to very flawed speakers. So we are dealing with an issue of someone with an audio reference point out on an extreme spectrum
However there is an issue in all this that comes up here regularly.
Speakers used to have large drivers like your Technics and wide cabinets. Now speakers have one or more small drivers and narrow cabinets. This is far from driven only by aesthetics. If you have a large baffle then you have a lot of reflections the cause peaks and nulls in the response, and as you have noted leads to poor imaging.
However the narrow baffle leads to progressive bass loss according to this formula: -
<small>f[SUB]3[/SUB] = 380 / W[SUB]B[/SUB]</small>
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(Wb is the baffle width in feet.)
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So this allows you to work out were the frequency tapers at 6 db per octave for any given speaker. This f3 point, where the response is 3 db down, occurs at the transition point where the speaker changes from a half space to a full space radiator.
Now the crossover should be designed to compensate for this loss. This comes at a price, and the price is progressive drop in impedance to allow more power to be drawn from the amp to compensate for this. For narrow front cabinets this requires a power increase just in the range where acoustic power demands are highest in the first place. This demand is often more than a lot of amps can cope with, especially lower end receivers like yours. In addition speaker cones are inefficient acoustic couplers to the room and smaller cones less efficient than larger ones as the frequency drops. The increase in power means more power in the voice coil and more heating, and therefore thermal compression in lees expensive and robust drivers.
Having said all that, B & W tend to have the most complete baffle step loss compensation. That is why in general their speakers present more difficult loads to the amp than other brands, with pretty much all their speakers dropping below four ohms where the power really is.
So you are likely to get improvement with a more potent amplifier, that is happy with loads at least to four ohms and a little below.
Classical music presents big problems, as the power required to reproduce all the cellos, double basses, trombones, tubas and tymps, largely falls into the range we are talking about. That is why really good speakers are such expensive and formidable creations. There are very, very few speakers than can produce the full wallop of a large symphony orchestra, or large pipe organ.
To do that you have be persistent and a few bubbles away from the norm by quite a few standard deviations. To that end I have constructed dual transmission lines to support the drivers over the two and a half octaves in a highly controlled and damped fashion. Baffle step compensation is provided not by passive crossover, but by active powered feed forward electronics. In these speakers, the smaller divers are spared the stress of baffle step compensation and it is handed to the upper 10" driver. Both 10" drivers are engaged below 60 Hz. Total power 750 watts to each speaker from three amp channels.
I took this measurement after high power conditioning and playing the test tones as loud as I could stand.
Even at that stress distortion is very acceptable and for a speaker very low in the bass.
The point is that for most, and me included away from this system, you have to dial back your expectations. However with your B & W speakers I would recommend much more competent amplification.
