ACG EQ-01 onboard preamp. One of these http://www.acguitars.co.uk/acg-eq01-filter-pre-amp/
Basically Alan Cringean and John East are geniuses.
Alan of ACG wanted to take some of the filter based goodness you get in Wal or Alembics, and imagine how that could work in the 21st century. Together with John East they made that happen and present a preamp that literally can do any bass sound in your head...
Control 1 = Top: Volume, Bottom: blend ... that blend is your best friend. Oh and it's not some passive blend like a lot of basses, but a proper buffered active blend.
Control 2 = Top: PU1 peak, Bottom: Filter freq ... So turn down the top knob all the way to begin with and start with the bottom - you've basically got a sweepable LPF letting you choose when to roll off the top end. The top knob is adds a peak just before the cut off frequency. If you imagine that the response of a pickup tends to have a natural roll off and a peak before it you'll see how you can dial in a jazz bass pickup tone, or sweep somewhere else for a precision type tone... it's so so flexible. Of course you could just turn the freq down and boost all the way and make the building shake.
Control 3 = Top: PU2 peak, Bottom: Filter freq. ... so the previous control does one pickup... and this control does the other pickup completely independently. your mind boggling yet? So how could you use this in practice? Well imagine you set up a big fat bottom ending tone on the neck pickup, and then a middy high end twang on the bridge pickup... when playing live just leave these controls alone and use the blend control to tweak the tone to taste. Oddly I ended up using the neck pickup really open with little boost, and then the back pickup with a lot of boost and kinda nasally mids, sounded awful on its own but blended into the neck it let the tone come through in the mix without loosing the bottom.
Control 4 = Top: HPF volume boost .Bottom: HPF freq. so you've got two pickups with low pass filters on them, and you've just got cut the highs on both of them... so all you've got is low mids and bass... which might be good for Reggae but... how do you add in some top end? Well we've got a High pass filter which you can add the treble in. Set the frequency to add it in from, and then the volume. In practice on a gig it's simple... you've got the freqencies all set up, use the blend to taste, and if it seems a bit dark, turn the top knob up.
Inside.... If that's not it there's some other controls inside... firstly volumes for each filter.... which is genius... that big fat neck Precision pickup can be the same volume as the weedy jazz pickup in the bridge when the blend control is in the middle. You will wonder how you lived without it! and then a blend control for the hpf.... select which pickup it's coming from. For example a magnetic pickup and a piezo pickup's top end will sound very different and you'll probably prefer one over the other. (NB I can't remember if this will do piezo or if it needs another board to do that.)
So yeah it's pretty cool. I've tried one on a fretless and it is amazing how it lets you dial in the exact mwwaaah sound you want. On a fretted bass it's super flexible. The tone it gives is clean and clear and very harmonically rich, especially into a good preamp. Playing you don't notice, but I heard a recording of something we did live once and it sounded very very Wal like tonally.
Why am I selling it... the bass this was in isn't my number 1 any more... and I've switched to helix and IEM and honestly with all the options of the helix, for a backup bass I craved simplicity. It is more complex system than say a P bass, but it's learnable so you can tweak it without thinking live and get the results you want.
We need to talk about knobs: John East has his own range of knobs which are very nice indeed. They would look wrong on a Warwick so I got these instead from somewhere, can't remember where. They are nice, solid and made of metal. From the weight maybe not solid brass, but you won't have any knob complaints quality wise.
Price is £165 posted to your front door in the UK. International at cost. Offers accepted.
Trades probably not really interested in at moment unless you've got a 500-800w "clean" amp (not mark bass) of some kind...