This Stereophile review describes the differences between the 331 and the 334, the .5 goes part way between and could be a DIY procedure. The sonics of the 331, 332 and 333 series was very similar . The 334, 335, and 336 series were more refined and more power than the previous series.
http://www.stereophile.com/solidpoweramps/142/
This should be what you need, lustformusic.
http://www.stereophile.com/solidpoweramps/142/
This should be what you need, lustformusic.