At 22 x 14 this will fit in a 1590B if you file the sides down a bit, but with it being a two stomp effect you'll probably want to put it in a 1590BB or at least a 125B anyway. But it could be squeezed in a 1590B, Box of Rock style, for someone brave/masochistic enough.
Pre Input and Pre Gain 2 are the connections for the Preamp stomp, Input and Level 2 for the main effect stomp, so each can be switched independently.
I have done this as per the scheme of the original, but a few mod suggestions for those that way inclined from D-Day on FSB:
The only way I could get the gain right was to put the tone section before the second op-amp exactly as a regular TS. It also seems to sound better without the 1M from the base of Q2 (mpsa18) to VB. I decreased the 1.5K from pin 1 to 5 to 1K and preferred it. Lastly it seemed to sound best with .47 coming from the clipping arrangement to ground instead of VB.
To use a regular TS style tone control, you would take the Tone 3 connection straight to Level 3, and you would need to remove the EQ Shift switch and the 100n and 3n3 which connect to EQ Shift 1 and 3. Then pins 5 and 6 of the IC connect to Tone 1 and 3 respectively (you will have to alter the row you use for the Level 3 connection), and Tone 2 connects to ground via a 220n cap and 220R resistor in series (or alter the values based on the frequency response you want). We have enough dead on TS clones though which is why I've left it as per the original, but there's the info for those who like to play. :o)
Info about the original:
The Hematoma is a two channel bass guitar pedal consisting of a Pre-Amp and an Overdrive. Pre-amp and Overdrive can be used independently or combined. Carefully constructed circuitry allows the original bass tone to pass through unaltered. No loss of low end. Pre-amp Features: Pre-gain control for up to 5db boost. Overdrive Features: Gain, Level, Tone and EQ Shift toggle which changes the sweep of the tone control and provides a warmer smoother tone when engaged. True-Bypass Switching.