Durham Sex Drive: This is a "clean boost" pedal that includes compression. Actually it's not 100% clean in the clinically transparent sense--just like most other boost pedals for guitar, it does add some slight coloration even at low-gain settings, and at higher gain it gets gritty, with more harmonic content. But it's not meant to get as dirty as a full-on overdrive effect, at most settings.
The compression only has three settings: soft, hard, and off. The hard setting is a high ratio, maybe around 8:1--good for tapping and sustain, but not quite enough for hard peak limiting. The soft setting is a low ratio, maybe around 3:1--best for more natural amplike compression and fullness of tone. The threshold for both is fairly low (relative to a typical guitar signal) so the hard setting will level out your dynamics significantly. Although the input gain control does affect the amount of your signal going into the comp, therefore acting as a threshold control, it also affects the amount of drive coloration in the tone--so there may be some instances where you wouldn't be able to get the exact blend of dirt and compression that you'd want, without boosting or cutting your levels before going into this pedal. But there is a good functional range just using the gain knob, for most instruments.
I'm not sure of the type of compression circuit here, because the circuitboard is buried in a thick chunk of epoxy, to prevent copying. Unfortunately that also means you can't fix it if a component ever goes bad. However it sounds and acts like an optical comp: smooth, with a mellow attack, and no exaggerated dips or swells.
The boost stays pretty clean up to about the 12:00 position of the gain knob; above that, it starts to get more textured, with more harmonic content. With the gain at maximum, and the comp set to "hard", it reaches overdrive-effect territory, with plenty of jangly gritty drive. But at most settings in between, it is more about giving your tone extra chime and body and edge, rather than obvious clipping. Durham describes the tone pot here as a "presence" control rather than the usual treble cut/boost. To me it seems like the tone controls the amount of upper harmonics added, so higher tone settings mean both more brightness and more gritty texture. But the tone is pretty bright overall, at any position of the tone knob.
There is no loss of highs at all; the low end is rolled off gently, meaning it doesn't actually lose any lows, but they do get attenuated a bit the lower you go. It sounds fine even on a bass low B, but it is better for someone who wants a tight, non-muddy tone, rather than somebody who wants huge dubby lows. There is hardly any noise at most settings--it only got kind of noisy with both gain and tone knobs at maximum.
The construction is solid and clean, and it runs on standard Boss-type 9VDC. Note the older ones are in a medium-sized box (like Diamond, Barber, BBE) and the newer ones are in a smaller MXR-type box. The footswitch is not "true bypass", but the bypass is excellent and clear.
Although there is hardly any control over the compression, the quality of the effect and the tone is so good that this has instantly become one of my favorite pedals. With the gain around 2:00 and the "soft" comp setting, I would be happy to leave this pedal switched on all the time.