Yo elove! I'm not really familiar with your units, but I have a lot of experience using outboard preamps with standalone digital recorders. Here's what I *can* tell you. First of all, your preamp puts out a line level signal. There are 2 versions of *line* level, -10dBV, and +4dBu. -10 is consumer line level, what you would expect from a CD player, home stereo, or minidisc. +4 is pro level, what you would expect from most dedicated studio recording gear. -10 is often transmitted by RCA jacks, and +4 by 1/4" or XLR. Either the XLR or 1/4" outputs can be "balanced" or "unbalanced", which use different cables. I'm pretty sure the ART MP puts out a +4 balanced signal by XLR or +4 unbalanced by 1/4". For your purposes, the balanced XLR out is the better choice, although either would work.
Now, the Boss could get tricky. First note that "mic" level is *much* weaker than either version of line level. By plugging the XLR line out into an XLR mic input, you are overloading the mic preamps big time, and you could actually blow up a mic preamp that way!!! You need to connect the line out to a line in. In this case, you are looking for a 1/4" (+4) line in, not an RCA (-10) line in. Your Boss may have both. A lot of recorders have -10 line ins so you can capture audio from a CD player, etc. To keep the signal balanced, you will use an XLR to 1/4" TRS (tip-ring-sleeve) patch cable, which has a 3-contact 1/4" plug, not a TS (tip-sleeve) cable, which has 2 contacts, and looks like the end of a guitar cable. An XLR to TS is an unbalanced patch cord, and an XLR to TRS is a balanced patch cord. The cables are not really balanced or unbalanced, but the boxes *are*, and a "balanced" patch cord will allow you to transmit the balanced signal.
Here's where it gets tricky. A lot of standalone digital recorders use variable impedence inputs, so as you increase the input level on the channel, it gradually changes from a line input to a mic input, and I don't know if your Boss has those. If it does, most likely, the input trim knob for that channel will say "line" all the way counterclockwise, and "mic" all the way clockwise. Or- there may be a switch on the back that says "mic" and "line". It is also possible that there may be a switch for the line input that says "+4" and "-10". If you have any of that, set it for "line" and "+4". You will turn the input trim on the Boss *all* the way down, and get your gain entirely from the outboard preamp. The purpose of a mic preamp is to raise mic level signals to the much stronger line level.
Now, all that said, there are 2 questions remaining unanswered. The first one is- will it sound any better than just plugging the mic into the Boss? Answer- only if the ART MP is a better preamp than the ones in the Boss. You'll have to make that call yourself, but they are both cheap preamps, and may turn out to be about the same (or not). The second question is one you asked- will this bypass the preamps in the Boss? Answer- no. The signal will still pass through the Boss's preamps. It will, however, greatly reduce the effect of the Boss's preamp on the signal, because you are not using it to produce gain. Usually, the more a cheap preamp is cranked, the more it sucks. You have 2 cheap preamps, so you may get your best sound in the end by using a little gain from both, so neither preamp is cranked too much.
Don't worry about the mono vs. stereo thing. The ART is a *one* channel preamp. It puts out a mono signal. If you send the signal to 2 tracks, it is the same as if you recorded on one track, and copied it to another. It's just a waste of a track, in most cases. There are a few tricks where you can process 2 identical tracks differently to create fake stereo, but real stereo recording requires 2 mics on 2 different tracks. Right now, let's just get the damn things patched up.
Last point- the Boss has an RCA jack(or maybe 2) labeled "S/PDIF". This is *not* an analog line in/out. It is a type of digital output (or input) that uses an RCA cable to transmit a 2 channel digital signal. It will only talk to another digital device that is synched with it, and is S/PDIF capable. Don't plug the ART into it. That would be bad. Hope this helps. Let us know if you figured it out, and which combination of preamps produced the best sound.-Richie