It would be helpful to know about your speakers, your listening room (size, distance you will sit from the speakers, etc.), and whether you will be using a subwoofer with the system. I can still offer several things to consider, and if you answer back with the info above, I'm sure other readers will be able to "fine tune" the advice.
1. I have no personal experience with the Meridian, but have extensive listening time with the Aragon, and my HT setup currently includes a Bryston 4B-ST and 5B-ST. It is likely that both the Aragon and the Bryston 6B-ST will do a fine job for you, although this is a bit dependent on your speakers and audio preferences. The Aragon is, in most systems, a smidge warmer than the Brystons, but if your speakers run toward the warm side (like Vandersteens do, for instance), you may prefer the Bryston. The 6B-ST, as you know, is the 3-channel version of the 4B-ST, which I have found to be a fine amp, particularly at the price. (The Brystons also have a great 20-year transferrable warranty, and perhaps the finest customer service in the business.)
2. The power rating on the Brystons tends to be quite conservative. The 4B-ST and 6B-ST amps are rated at 250 wpc, but typically are in the 290-300 wpc range.
3. If your center and/or surround speakers are above average in efficiency (above 89-90 db), you may find that the Bryston 5B-ST will fill your needs. The general consensus on this forum is that surround speakers often do not require as much power as the main front speakers, particularly if the surrounds are smaller, monitor size units that do not have much output below 60-70 hz. However, if your surrounds are floor-standing, full-range speakers, you should provide as much power to them as to the main front speakers.
4. If you decide to go with the Bryston 6B-ST, drop me a personal E-mail and I will send you information about the dealer where I bought my Bryston. He offers a substantial discount, and sells the 4B-ST for about $1650 (vs. $2395 MSRP), and will probably sell the 6B-ST for about $2400.
I hope someone who knows the Meridian line will respond to your post. They make fine gear, and are certainly worthy of serious consideration.