Welcome.
The Meridian processor will be a lot more refined and smoother than the Tag processor, I had an AV192r for a few days after being really impressed with a demo against a Meridian 568.2 (which I owned at the time), the Tag was faster, livelier and more exciting.
However after 3 days I took it out, realised that it was not offering anything above the Meridian, the Meridian actually had more detail, more refined, more natural and smoother, a lot more grown up.
I ran my Meridian processor with amps and passive speakers for many years, around 6 in total, using Classe and Cinepro amplification and Dunlavey speakers and loved it.
However, I went DSP last year, I actually did it as a lifestyle choice, and was prepared to take a hit in quality for a nice form factor and wife pleasing design, however I have been blown away with them.
They have exceeded anything passive I have ever owned and they are only the 24/96 DSP5000s (used with a Rel Studio sub).
No demo ever revealed to me just how good the DSP speakers are, like you I always thought they sounded like passives of around the same price. It is when you get home and use them and set them up properly that they start to show you what they are about.
The fact they are DSP and sound the same played at -40db and -0db is worth it alone for me, as I listen to around 4 hours of music every night after 10pm when you have to consider the family and neighbours.
SW1600 with 7200's is a waste, the 7200s will do it all.
I would get 3 x 7200s.
I would also seriously weigh up if you need an 861v6 and HD621 at the moment, when you consider you can get a non speakerlink G68 or G61r for �2000 it does make the 861 a bit of a jump.
If it were my money I would get 3 x 7200 + 2 x 5200 and a G61/8 + HD621.
Then get the 861v6 when funds permit, and loose the sub completely.
Depends if you listen in stereo, or Trifield or watch alot of films.
The Meridian surround experience is like nothing else I have heard, I went from Meridian 5.1 to Lexicon 7.1 and back again, and then from Meridian 7.1 (matrixed rears) with passives to 5.1 DSP and the 5.1 DSP beat the lot.
So far all the best systems I have heard have been 5.1, not saying 7.1 is bad, all things equal I would always have 7.1 if the room takes it, but I would not move from Meridian to get 7.1.
The reason I mention this is the main advantage of the 861 for me is 7.1 rather than 5.1 with matrixed rears.
I see you are 5.1 but may be important at some point?