I have used multiple xti2000 and 1000 exclusively for monitors for the past 3-4 years, and its fine with 4 speakers. Only at one time did I have a fault, when I tried (4) 15" monitors with Radian horns, which I suspect were either rounded to 8 ohms, mislabeled, or modified.
A little tip however try to get the newer ones if you are buying used. I had weird issues with pre 2006 models. Random volume spiking and every now and then, one Chanel suddenly not working. (These were also both 1000's though) look for model numbers above the ones on this page,
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Both the units I had problems with needed the firmware update. they were less erratic after the update, but still did not perform properly after the update.