I installed a new attenuator the other night in my Bottlehead S.E.X. and have now had a chance to listen to the setup for a little while. There's no doubt that it's a significant improvement. As others have reported, the key differences from the stock potentiometer are better transparency and detail. I feel like the textures of recordings and things like breathiness in vocals, etc. are now more accurately and clearly rendered through the S.E.X.
My attenuator of choice was a Chinese stepped attenuator assembled using high quality Dale resistors. The attenuator arrived from the eBay seller with some minor damage due to being crushed. The wire ring around the bottom of the attenuator was bent and snapped between 2 of the resistors, but was relatively easy to fix with some gentle bending and then some solder. Hopefully it will hold up in the long term, but I at least I'll know exactly where to look first if there are ever any issues. The seller kindly provided a partial refund to cover the inconvenience.
The new attenuator is a little bit stiffer to turn than I like, but I can deal with that in return for the more open sound it's providing.
Here are some pics. The first one shows the attenuator design and you can see the wire ring that broke in transit (at the top of the attenuator in this image)
In order to install the attenuator properly I had to grind out a little bit of extra length and width in the chassis slot for the locking tab on the attenuator.
The stock volume knob now sits a little high so I might go looking for a vintage looking knob with a slightly recessed underside. If anyone knows good places to source vintage-looking knobs, please let me know.
The fly-leads on the attenuator made connection really easy because I wasn't limited by the length of the existing cables. There are now a couple of loops of extra cable hanging down from the chassis, but they don't extend much further than the height of the C4S board so it's not at risk of being snagged on anything and it's not visible.
In addition to heat-shrinking the connections between the existing wiring and the attenuator fly leads, I also wrapped each bundle (left and right channels) in electrical tape to protect the drain wire connections from the stock wiring configuration.
I can highly recommend this modification. My assembled attenuator was just $50 from eBay and is easily the best improvement I've found for a Bottlehead kit other than Bottlehead's own upgrades (Speedball / C4S). Of course, this upgrade would not work (nor be required) for the Mainline, just the Crack, S.E.X., or Quickie.