Quad ESL Cap mods

I believe it is a coupling cap, acting as a high-pass filter (filters out the deep bass), therefore it is very important for sound quality.
220uF in film caps will be very costly and occupy a lot of space, about the size of four soda cans. The least expensive option is four Solen 47 uF / 400 Volts caps plus one Solen 20-30uF uF / 400 Volts plus one Mundorf Silver/Oil 4.7 uF, all in paralell.
Since the original electrolytic cap has a tolerance range of +/- 10%, this combo will be in the ballpark.
You may want to do the full combo with Mundorf Supreme caps and use a 4.7 uF Mundorf Silver / Oil as a high frequency bypass cap.
There are many cap comparisons on the net, most agree that Mundorf Silver /Oil is one of the best caps regarding clean and extended treble.
I have experimented Mundorf S/O as compared to Kimber, Auricap, Hovland and Jantzsen film caps in the tweeter network of my dynamic speakers, all those caps sound good but Mundorf is ahead of the pack IME.
I hope this helps

William, I have no experience with Mundorf Tube caps, because Tube caps are designed for high voltage tube gear power supplies, not coupling.

Regarding Face's Mundorf cap, it is a great choice if the original cap is rated for 250 Volts or less, however, I maintain my recommendation to bypass any large film caps with a Mundorf 4.7 uF silver / oil. This will provide speed and transparency in the treble range, which large film caps tend to lose due to high internal inductance (side efects of the soda can size).
224.7 uF as compared to 220 uF means that your speakers bass response will extend a couple of Hertz down, not really a factor given the usual 5% tolerance of large film caps and any resistors used within the speakers.
Good luck

HI
I have Auricaps and Mundorf PP caps in my tube amps (not the same application, I know). I prefer Mundorf Supreme to Auricaps IMS.
Mundorf caps are metalized polypropilene.
Munforf silver and oil is a significant step forward in transparency as compared to M. Supreme or M. tube cao, but it is only economically feasible as a bypass cap.
Many high-end speaker manufacturers are switching to Mundorf caps for their top model's crossovers: BW 800D, Magico, ZU Definition and some others I forget.
Any cap smaller than 220uF will attenuate deep bass response.

Regarding the paraleel resistor, find out its value and power rating (usually written in the transistor body).
Mills MR12 is widely well regarded as a neutral, non inductive resistor up to 12 Watts per resistor. You can use several in paralell to reach the correct ohmic value and power handling. E. G. ten 80 ohms 12W resistors in paralell will add up to 8 ohms and 120 watts.
Dale makes aluminum-clad non-inductive resistors in 20W and 50W ratings.
DO NOT install the resistor flush with the cap(s), because it will get very hot on music peaks and may damage the caps.
Any PP or MKP cap will certainly sound much better than an electrolytic cap, but if you can avoid the cheapest ones, it will be a wise investment, given the Quad ESL's transparency and midrange naturalness.
Good luck

I hope this helps

Hi Mksj
By all means remove the original resistors and capacitors if you perform this mod.
Also, make sure you buy the right wattage resistor. I suspect that a lot of power flows through the resistor at high volume level.

Mksj:
I believe that the main reason why you are delighted with the sound quality after modding the speakers is the difference in distortion, clarity and speed between crummy electrolytic caps and high quality film caps.
I still can not believe that QUAD had the nerve to use lytic caps in series with the signal...Quad 988s are not exactly budget products.
A cap with unmeasureable ESR and DA is electrically pretty close to a straight piece of wire (within its frequency pass-band).

Also, your ground mods were a smart move. Many designers tend to think of ground as a zero volts, zero current wire, supposedly not important at all.
In practice, because of ground wiring resistance, voltage and current ride on the ground wiring and mess up the signal return path.
Grounding layout, wiring and connections are very important and are very rarely implemented correctly.

ENJOY!

Sugarspice, I think that protection at the speaker is safer.
Many well-regarded amps thump when powered up or down, not to speak of the absent-minded user that turns the amp on, then the preamp.