I agree with a lot of things you say, but it's just that the sentence "A lot of people out there feel that Markbass gear sounds better than the Sisma." is excessive IMHO because very few people have actually tried Sisma since it's hard to find (so how can there be "a lot of people" since very few actually know it), and because I've never seen anyone saying he prefers MB or Sisma - that's all. So I wonder: maybe you see yourself as "a lot of people"?
My personal view on Markbass amps VS Sisma (not saying it's objectively better, just why I prefer it):
- Markbass sound really good IMO, especially with the filters at zero, I find them transparent but not cold, simply good sounding. Like @Nuisance, I prefer MB amps than their cabs. For context, I don't own one, but I play them very frequently in rented rehearsal studios.
- I don't like MB filters, but it's not an issue at all — I just avoid them.
- The EQ section is efficient, but nothing crazy
- I prefer Sisma for the following reasons:
- I prefer Sisma's EQ, with the Bass and Treble controls which act on a wide frequency range, super quick to use, and the chirurgical parametric mids than can fix any issue.
- Sisma can sound clean and almost flat if desired, or much more colored depending on the settings, and it never sounds bad for my taste, whatever the settings. By comparison with a MB head, there are settings that I dislike and situations where I struggle.
- There's something glorious with Sisma when you push the master volume. I don't know if it's due to the Class G power section, maybe it would sound exactly the same with a classic Class D power section - I don't care about the technical reason. But it's something I've never experienced with a MB amp. When I set the EQ at low volume and then increase the master volume, the tone remains the same (it doesn't become boomy), and with the added loudness comes an extra "je ne sais quoi" that I (and my band mates) can feel. I know it sounds like BS and I'm sorry for lack of better words, but it's hard to describe a feeling (+ English is not my native language).
That was just my personal opinion.
Like you, I prefer the dirt of the CAPO than pushing the internal tube of Sisma's preamp, but it's the same with all the tube preamps and full tube amps that I've tried, nothing specific to Capo and Sisma. It took me years to find out that I don't care for real tube saturation (at least for bass), I prefer transistors saturation in general. But I love the sound of tube when set "clean, at the edge of breakup". And in fact, I keep Sisma clean and use Capo for distortion. But Capo followed by a clean Sisma sounds better than Capo alone, for my taste.