Short answer: probably not much of an advantage.
You can stop reading here if you want
Since you ask, though, the best option would be to get an XLR-to-TRS lead, plug the XLR end into the Spectradrive and the TRS end into the Scarlett, and set INST/LINE to LINE (so the light is off). This is what I do with my own amp simulator and audio interface.
There's a lot to unpack in that sentence, so here's the longer version:
XLR and TRS (Tip-Ring-Sleeve) are two types of plugs for balanced connections. I have no idea what XLR stands for; you'll often see it referred to as a microphone plug (or socket) because that's its original purpose. TRS looks like a regular jack plug for an instrument lead, except that it has three conductive sections instead of two.
"Balanced" means that instead of just having a signal wire and a ground wire, you have
two signal wires and a ground wire. The signal wires are exactly out of phase with each other, so when one is positive by some amount, the other is negative by exactly the same amount. The point of this is to cancel out any electrical noise that the cable is picking up - it's the same idea as humbucking pickups.
But this isn't why you want to do this, unless you're getting lots of noise from electrical stuff around you, like fluorescent lights.
The reason you want to do this is that the XLR out on the Spectradrive gives you a line-level signal. This uses a higher voltage than the signal your instrument produces, which is why there's a line/instrument switch on the Scarlett. Importantly, it's a
standard voltage level, which everybody engineers to. This means you don't need to fuss with getting the right input gain level for your instrument's output, which is convenient.
The reason you need to use a TRS plug on the Scarlett end is that the XLR socket is set up for a microphone - a line-level signal will be too loud, so the result will probably be lots of distortion of the wrong kind (we call it clipping, because the signal is "clipped" off at the interface's highest level).
The Scarlett's line-level input is the jack socket (input 2). It's designed to accept a line-level signal, as long as the line/inst switch is set to "line".
If you're still with me after all that, the answer to your second question is yes: you could do the same with your BH550.
If any of that didn't make sense, I'll be happy to clarify. There's a lot to learn quickly, in here
