I can't speak to whether you will hear a difference between the Adcom GFA-555 and the Marsh 200, since I have not heard the Marsh 200. My guess is you will hear an improvement, since the Marsh 400 has gotten very positive critical reviews, and the Marsh 200 is just a lower-powered version of the 400. Obviously, the best way to compare amps is to conduct an in-home audition.
I can, however, comment on the Adcom GFA-555 vs. Bryston 3B-ST amps, since I have substantial experience with both of these amps, and with your Vandersteen speakers. The Vandersteen 2c speakers are sufficiently revealing that they benefit from improvements in amplification, and with the 3B-ST you should hear better controlled and defined bass, significant improvement in mid-range clarity and transparency, increased transient speed and dynamics, and better soundstaging and imaging. (The Bryston web site -- www.bryston.ca -- has reprints of all of the reviews of the 3B-ST done by various audio mags. If you have not read these reviews, you should do so.)
In addition, the 3B-ST should be a felicitous pairing with your ARC preamp. The 3B-ST has vanishing low harmonic and IM distortion and a signal-to-noise level that is far lower than the SP-9 (lower, infact, than CD's). (If you compare the distortion and SN levels of the 3B-ST to the Adcom, you will find that the Bryston's levels are typically 1/10 to 1/100 that of the Adcom.)
The 3B-ST is rated at 120 wpc, but actually produces about 150 wpc. The 3B-ST's minor decrease in power from the Adcom 555 (200 wpc) will not be audibly detectable (to be audibly detectable, there would need to be about a 2:1 difference in the power output), but you will hear the improvement in the overall quality of audio reproduction. The 3B-ST is one of the finest reasonably priced, mid-powered amps on the market, and the 20-year transferrable warranty is, hands-down, the best in the high-end industry.
There have been several used 3B-ST amps for sale recently here on Audiogon, and for around $1200 you won't do any better than the 3B-ST. If you want to buy a new unit, drop me a private E-mail and I'll give you the name of a dealer that offers discounts on Bryston gear.
I can, however, comment on the Adcom GFA-555 vs. Bryston 3B-ST amps, since I have substantial experience with both of these amps, and with your Vandersteen speakers. The Vandersteen 2c speakers are sufficiently revealing that they benefit from improvements in amplification, and with the 3B-ST you should hear better controlled and defined bass, significant improvement in mid-range clarity and transparency, increased transient speed and dynamics, and better soundstaging and imaging. (The Bryston web site -- www.bryston.ca -- has reprints of all of the reviews of the 3B-ST done by various audio mags. If you have not read these reviews, you should do so.)
In addition, the 3B-ST should be a felicitous pairing with your ARC preamp. The 3B-ST has vanishing low harmonic and IM distortion and a signal-to-noise level that is far lower than the SP-9 (lower, infact, than CD's). (If you compare the distortion and SN levels of the 3B-ST to the Adcom, you will find that the Bryston's levels are typically 1/10 to 1/100 that of the Adcom.)
The 3B-ST is rated at 120 wpc, but actually produces about 150 wpc. The 3B-ST's minor decrease in power from the Adcom 555 (200 wpc) will not be audibly detectable (to be audibly detectable, there would need to be about a 2:1 difference in the power output), but you will hear the improvement in the overall quality of audio reproduction. The 3B-ST is one of the finest reasonably priced, mid-powered amps on the market, and the 20-year transferrable warranty is, hands-down, the best in the high-end industry.
There have been several used 3B-ST amps for sale recently here on Audiogon, and for around $1200 you won't do any better than the 3B-ST. If you want to buy a new unit, drop me a private E-mail and I'll give you the name of a dealer that offers discounts on Bryston gear.