I strongly agree with your first two points, but disagree with your third. 0dbfs today is the same as it was 5 to 10 years ago.
The input of your M-Box is a DI. It converts a high-z input signal to a low-z output to feed your preamps and converters. If your bass sounds "king of weak and flat" the M-Box isn't the problem. You might see some minor tonal change (read: not necessarily an improvement) by using a higher end DI box, but it's not going to be night and day.
As to your original question: Outside of the excellent advise you've already received from @And I and @Badwater I would suggest a very specific EQ primer. Once you've removed the unwanted "stuff" (too deep lows, and too extended highs) you should zero in on the upper midrange. Use a spectrum analyzer to correlate what your ears are hearing. Bass in a mix needs more upper midrange than most people typically think if you want it to sound present and detailed. Bass on its own doesn't have to fight through guitars, cymbals, drum attack, vocals, etc. and as such can get away with less going on in the upper midrange. If you find things sounding overly aggressive, excessive amounts of string/finger noise, ganky, clanky, etc. you can use a narrow Q cut in the upper midrange to make things more pleasant in this solo context.