I'm sure your mid-lo bump, if there is one, comes from something other than Markbass. I refer you to the graph of Markbass response with VPF at max. The graph shows a boost around 40 Hz, a deep notch between 300 and 400 Hz, and a significant boost around 9 KHz. No bump. When VLE and VPF are off Markbass amps have a very flat response.Fair enough, you are right in that there is a mid-lo bump (especially with the VPF turned up). I suppose it would have been more accurate for me to say that if Ampeg is your gold standard of bass tone and you love that sound to death you will be disappointed with MarkBass. They are lightweight and plenty powerful, but they won't give you a tone like an Ampeg.
Markbass amps are often called "sterile" because their flat response curves (VLE & VPF off) don't color the music as other amps do. e.g. The characteristic traditional scooped Fender sound. If your ear is attuned to a scooped sound a flat response may sound bumped.