I have owned both the Rowland Concerto and Continuum 500 integrated amps. Both were stellar performers. The Continuum 500 was very refined, very transparent, portrayed an excellent soundstage with extraordinary imaging.
I actually drove a pair of extremely power hungry Eggleston Andra II speakers, as well as a pair of Sonus Faber Guarneri Mementos with the amp and the performance of both pairs of speakers was extremely good. I sold the Rowland to purchase a McIntosh MA7000, just because I wanted to "try" the Mac. This was a lateral move at best. The Mac is good, but I certainly don't think it to be "better" than the Rowland.
The Rowland "right of the box" was pleasant to listen to, but it improved dramatically over the first 200 - 300 hours of use - to the point that it equalled most separates of my experience.
I can't give you any direct comparison to the Levinson 383, but I will say that, to me, it was a one-box equal to the Levinson No. 32 preamp and No 336 power amps that I used for many years. It is that good!! It is also very close to giving equivalent performance to Spectral separates that I currently use in another system.