Hi Rinpoche, SS vs tubes is not the best way to look at the problem of amps... Rather, the thing to look at is the actual sound of individual amps, or at least the house sound of particular brands. Hence, you are doing well in your current path to final choice... One Rowland SS, and one VAC tubed... Yes, definitely you will find marked differences between the two amps... Yet their audible personalities have much more to do with the sonic phylosophies of the two designers, then about one amp using SS and the other one tubes.
Good idea to evaluate amps or any other component always with the same set of recordings... I have been using the same 6 audio tracks for the last 15 years... THis has helped me to create a stable frame of reference.
I use mostly the following pieces...
Antonin Dvorak -- In the Old Castle (from a 4-CD set of all Dvorak piano works on Brilliant Classics CD) with Anna Poroscina playing a a Bosendorfer grand... I look there for harmonic exposure and stability of the piano tone across the 7 octaves, naturality of attacks, persistance of decay, and extent of low level information (sounds emitted by the performer, felt noises, piano bench).
Antonin Dvorak -- introduction to string sextet with double bass. The introduction has some passages with the higher string parts bunching together in the mid treble during the cusp of cadenzas.... Many amplifiers cannot handle such closely scored sostenuto in the treble region and break apart: The sound becomes shrill, strident, and unlistenable because of intermodulative artifacts.
One more Dvorak... 2nd movement of the Symphony from the New World Op. 95 with the Israel Phil under Leonard Bernstein... My favorite performance of this work.... Look at the following:
* How does the amp handle the opening fanfare for lower brass...
Can you hear the individual horns and trombones, or does the image become homogeneous and an indistinct musical pudding?
Gradually the brass section shows some cuivre.... Does it remain coherent and musical... Or does it exibit signs of intermodulative strain?
Are the timpani pitched or tubby?
How well can you hear the hall in the low level information?
How clear is the double bass line.... deep and well pitch, or surfacy and tubby?
What happens in the ff at the end of each section? Does the amp sail through without effort with all instruments still clear and sweet.... Or Does the image seem to saturate with a front-to-back collapse of stage and images, and the appearance of hardness?
Can you hear Bernstein stomping his feet to encourage the players some 12 minutes into the movement?
Can you hear him subvocalizing during the final brass fanfare in the movement?
Few amps can do these things correctly even if they are fully broken in. The M625 original version (except for the very first few serial numbers) negotiated all the scenarios above without any issues... The S2 version is much improved over the original.
I do not have sufficient experience with the VAC amps to make a meaning ful comparison.
Regards, Guido