Rich, I don't know their newer product line very well (I assume you're looking to buy new as opposed to used). 

The nicest sounding Marshall I've ever played through was an old JCM 800.  They also get the prize, I think, for being the Marshall that's most often modified (go figure).  My 900, I think, would sound good as a head & cab, but as a combo it sounds brittle.  I was also, once upon a time, thoroughly impressed by their MOSFET 3/4 stack (GASP - no tubes!).

Let your ears be your guide.  Generally, those things don't start sounding good until they're turned up nearly halfway.  At that point, they're moving lots of electrons!  Your wife, kids, neighbors, etc. might wish you had bought something else.  As a rule, they're also very directional - I'm not sure why.  The difference between standing right in front vs standing 10 degrees off to one side is enormous.

At any rate - have fun shopping, and keep us posted.  The only thing I can think of that's better than you getting new gear is me getting new gear

*sigh* I'm still hunting.

SG

There are a lot of different types of Marshalls and the model that may be right for one person, may not be right for another.  It depends on what kind of music you are going to be playing thru it, and the guitar that you will use.

Here is a run down on some of the Marshalls.

JTM45 is a 30W head.  This is essentially a Bluesbreaker amps in a head.  The Bluesbreaker amp is a combo that Eric Clapton used on the John Mayall and the BB album.  It was the first  time someone had played a Les Paul thru a Marshall.  The JTM45 is powered with 5881 power tubes.  This is a very sweet sounding amp and would be ideal if Blues is the flavor you are after.  They are very loud considering the low wattage.  This would be best paired with either 25W breenback speakers or Vintage 30's.

1987/1959 These 2 amps are RI of the famous Plexi amps of the mid to late 60's.  These are the wholly Grail of Marshall collectable amps.  The RI are probably not going to sound as good as the original, but they are really nice amps. 1987 are 50W the 1959 are 100W.  These are powere with EL34 power tubes so the have more bottom to them than some newer amps.  These amps are incredibly loud and don't really begin to work well until you crank them up.  Both of these amps do not have master volumes so you have to turn them up to get them to really sing.  The best amps ever for 60's style Rock.  Think of Hendrix/ Led Zepplin.  This is the Amp they used.  I prefer the 50W 1987 they tend to sound better to me.

Marshall 800.  The amp that so many people used in the 80's.  There are a few different configurations of these, but the one that most people prefer are the 2210.  Non master volume.  A little easier to get a good sound out of them than a Plexi, but at the same time the tone is not a s sweet.  This is a great amp in general..... espicially for 80's rock.

Marshall 900.  This in essence is an 800 with more control and more gain.  I personally feel the 800 is a better sounding amp.  I have a 50W Combo of this and I love it.  Its light.... easy to move has 2 12" speakers and the gain on  the amp is more than I will ever need.  A lot of people have a lot of negative info about 900's.  I don't know why I think they sound really good.  They do have a transistor preamp though, but as I said, they sound pretty good.

DSL 2000.  The DSL (Dual Super Lead) is a 2 channel amp that has really nice cleans reminicient of the Plexi, but easier to control the volume.  The Drive channel can go from Pexi.... to 800.... and far beyond.  This is an extremely versatile amp.  You can play blues to metal with this amp.  This is also available in 50W or 100. (I tend to favor 50W Marshalls over the 100's).  Each channel on these amps has 2 modes so it kind of has 4 channels.  I like these amps a lot.  There have been some reports of poor construction, but I personally have not seen anything that I would consider to be inferior build quality.

TSL is a fairly new amp and the few that I played thru I did not like. Its a three cahnnel amp. Clean, gain, and a whole lot of gain.   This is an amp that is best suited for High gain Metal type of work.  Not my thing, but if thats what you need this would be the amp to look at.

The speakers you pair with the heads have as much to do with the sound as the head itself.  I prefer the Slant Cabs with either Vintage30's,GT12H or the 25W greenbacks.  Lower wattage amps work real well with the Greenbacks, but the vintage 30's can sound really good with just about any head. GT12H are 30W speakers and also sound really good.  The bottom line is you need to get into a music store that has a variety of cabs and heads and you need to play thru as many different combinations to see which one will work best for you.

Here is a link to the Marshall website so you can look at the specs.

www.marshallamps.com