Gradient SW-63 crossover upgrade & service

The Gradient SW-63 crossover unit is provided with the Gradient SW-63 subwoofers, an ideal companion to the Quad ESL-63.
However the crossover unit suffers from a number of design flaws:
1. Phase incoherency around the 115Hz crossover region, leading to cancellation between the subwoofer and the ESLs, rather than sound.
The cancellation act a bit like a notch filter around this frequency.
In effect the SW-63 and ESL-63 are partly driving each other rather than the air in the room.
The effect is a loss of signal around the 115Hz region, which is for example the 'meat' of the register of the male voice, cello, bassoon, French horn and trombone, lower guitar strings, piano left hand, etc.
 After the upgrade you will notice more volume, deeper and more solid bass, more presence in these registers, and lower noise and distortion.
2. Too many op-amps in the signal path. The low-pass path to the subwoofer contains four op-amps. This can be reduced to three, reducing distortion.
3. The 'mid' switch is fairly pointless and is implemented via an analogue 4066 switch which is not distortion free.
4. There is a power-on delay implemented via another another analogue 4066 switch which is fairly pointless as these units are generally powered up permanently. Mine has been on for 13 years excluding moving house.
In addition, all these units from the 1990s are now due for a service. Electrolytic capacitors should be replaced and op-amps upgraded.
In the upgrade, we:
(a) correct the high-pass filter from 2nd order to 3rd order to fix the phase incoherency and improve the cutoff below the passband.
(b) reimplement the low-pass filter with one less op-amp
(c) upgrade the op-amps if necessary to OPA2134. We are investigating premium-grade options but for example the very high quality OPA627 and LM4562 aren't available in a dual 8-pin package so cannot be offered.
(d) Remove both 4066 switches and disable the corresponding functionality. As an option we can upgrade the power-on delay switch to a low-distortion MAX4066 switch.
(e) renew all electrolytic capacitors.
High-quality audio-grade electrolytic capacitors, polypropylene audio filter capacitors, and low-noise 1% precision metal-film resistors are used throughout the upgrade.
Price: $99.95. 12 months warranty on all parts and labour.