Check
this out ... specifically #2.
I know absolutely nothing about tube amps... other than I have always heard they introduce distortion into the signal... and/or color the sound.
If you are looking for that softer high-end, you could equalize it out. If you find an amp that causes that warmer sound, it is doing something to the response to cause it to sound that way. I could have swore I was hearing a warmer sound from a Yamaha receiver I tried one time, but after further consideration, it ended up being the speakers I changed out that made the sound warmer... the speakers were more laid back in the top in. Response measurements showed the top-end response sucked out heavy... and it sounded really good to me.