Where to start !!!!
SS v Vale - yes it makes a difference. Valve power amps compress and even break up when pushed. With the AFX you generally dont want this as your getting those characteristics from the AFX. Also, SS amps have high OP impedance and vale ones low. That means the valve power amp reacts differently with the speaker dependent on the frequency of the sound. Again though - you get that fro the AFX so shouldn't need it from a valve amp/cab interaction. That said - for SOME people, that AFX sims arnt 100% yet - in particular that amp/cab interactivity when using a real cab, and thses players still prefer a valve power amp. It is more airy, and slightly scouped in general - but nothing that cant be EQ'd either way (from a tonal rather than interactive POV). The only Question with valve power amps, its amp sim on or off in the AFX. Personally when I used a VHT amp I left them on as I was never playing loud enough to even tickle the amp. Once you get to decent volume levels though you need the sims off - and that kind of defeated the point for me personally.
Secondly - watts. Dont be scared of 400 or 500W SS amps. A valve power amp will produce way more than its rated watts when pushed full up - often twice or more. It will also compress slowly - so you dont hear when the signal starts to distort (as opposed to clips - the signal peaks flatten slightly and thats when you get a warm top end with harmonics). With a SS amp you want to stay WELL AWAY from clipping/distortion as it just sounds bad. Given 3db is double power - and you really need at LEAST 3db headroom in a SS amp - the working watts is at least half, maybe even 1/4 (if you really want to play safe, and have maybe quite a big lead volume jump) of the amps rated output.
On top of that - you nee to look at your cabs ohm rating. With a valve amp it doesnt matter - theres generally a switch to match the amp to the cab and the actually watts output is the same. Not so with a SS amp. SS amps are generally rated into 4 Ohms, and every doubling of the ohms will reduce the power the amp generated by roughly 1/3 - and for some amps its 1/2. So - if you use a 16 Ohm cab - that 400W Matrix is actually only going to generate around 160W into that cab. Then take into account the minimum 3db (half power) headroom and your actually left with a working equivalent power of approx 80W. Given the valve power amp may well generate double its rated output - The TS100 could get up to 200W when pushed. This is often why people say valve watts are louder than SS watts. There not, a Watt is a Watt- its just how much watts are actually generated into a given cab in a given situation.
S0 - either the Matrix 800 or 1000 are perfectly suitable and roughly equivalent to a 100W valve power amp - into a 16 Ohm cab. Into an 8 or 4 Ohms there a lot louder and you do need to be aware of the power or you "could" blow speakers. Its easy to keep that in check with ah AFX though - just set you master OP gain lower to restrict whats coming out with the volumes max'd.
So - summing up (ask q's if you need). Tube or SS amp is personal, but in general the more updates cliff adds to the AFX - the less necessary a valve amp is to get the tones from an amp/cab set-up. Secondly, a 400-500W (per channel) SS amp is probably the minimum you need into a 16 Ohm 4x12. If your using a 4 or 8 Ohm cab you could drop that to 200W or so. The Carvin DMC200 you mention is fairly nice sounding, but IMO very under powered for a SS amp in this context. It wont be anywhere NEAR as loud (cleanly) as the TS100.
AS a guide, my VHT 50W valve amp was just louder than my Matrix GT800 when both were at maximum (without clipping). My GT1000 is slightly louder/ohm par with the VHT. This into am 8 Ohm 2x12.
