John,I did an analysis (see link below) last year on a similar McIntosh design (the XRT-29) and much or that thinking likley applies to the XRT-28. That analysis is down the page in this forum at:
http://audioroundtable.com/ArraySpeakers/messages/492.html
Now you can duplicate their design for less money but there are a couple of things to keep in mind. One is the relatively low crossover frequency between the mids and the tweeters given the symmetrical horizontal arrangement of the mids and tweeters. So the drivers need to be selected to satisfy these requirements. McIntosh doesn't use exotic world class drivers so you can likely find several that can meet your needs. McIntosh tells you a lot about their design in the owners manual.
Frankly, I design arrays differently vs. McIntosh as you will understand if you have read my white paper. I like planar or ribbon tweeters vs. dome tweeters.
Good luck and let us hear what you choose to do.
Jim