Both the Saturn V/VI and the Mercury IV could both be loosely thought of as boost-to-low/moderate OD pedals, but they differ in type/approach.
The M4 is transistor-based, basically a modified treble boost circuit to my ear, so its DNA will always put it a little more in the brash/crackly/crunchy camp versus warm/smooth. But the modifications make it so that you can run it as a very clean boost with almost no transistor artifacts. The “harmonics” are dialed in separately, not as much apparent “gain” (clipping) as some treble boosters would have (more sparkly like, say, the Aphelion), but massive output to push a tube amp into drive.
The beauty of the M4 is in the EQ switching. It’s like a range knob that is split into two sections that work independently. You can get a lot of really cool tonal variety from different switch combinations, either with the drive up or totally clean. For guitar, I liked the M4 best at the very end of the chain, as an enhancer type of thing. It’s also really great at running after fuzz where you may have a fuzz that just doesn’t punch through the mix well but you love the sound. Working the switches and such, you can make any fuzz punch through and sound just the way you want it to without neutering the character as many boosts and OD’s will do. I don’t have my M4 anymore, I have a 3-knob Arc Gamut that basically does all the same things and I had much more money sunk into the M4. (Decades long guitar player, I just picked up bass a little over a year ago, after the M4 was sold, so I have not tried it with bass.)
I’ve had both the Saturn V and VI, still have the VI (never tried the V with bass, I had/sold that one years ago). The Saturn is more of a warm, smooth boost or low gain OD with a bit of mild compression. There is some nice clarity - even in the midst of the drive, you can hear the strings. I like it OK, but it’s not really my favorite for either guitar or bass. The Polaris is much better to my ears, with the filtering there is a lot you can do, and it sounds great with both bass and guitar.
I’ve had most of the Spaceman pedals over the years, currently still have the Voyager, Nebula, Saturn VI, Polaris, Atlas, and Sputnik III.
Just purchased a used Delta, kind of excited about that one!
