I am a Sunn-O-Phile by nature, but also am a huge fan of vintage Hiwatt gear. I was at the Hiwatt convention this past weekend, not a Sunn in sight but Hiwatt amps everywhere. I have extensive experience with both amps, and I think that the amps share many attributes. Compared to the majority of tube instrument amps, both brands are
very Hi-Fi, both are very loud, both stay relatively clean at high volume, but they have different tonal characteristics.
Here's how they sound to me;
Hiwatts are usually characterized by the term "Chimey", they have a pleasing but distinctly glassy ring to their tone, and stay clean at volume levels that would have most other tube amps screaming for mercy.
Sunns are generally warmer and richer sounding, a more mid range presence, equally pleasing to the ear but different.
I think the Hiwatts have more headroom watt for watt, but both Sunn and Hiwatt are high headroom amps compared to the masses.
The differences are more in the circuit than the actual tubes, if you take a 6550 out of a Sunn and put it in a Hiwatt, it will still sound like a Hiwatt and vice versa.
By the way, DR201's came/still
come with KT88's not 6550's, which ever you use is just a matter of taste.
Since this post on talkbass.com;
http://www.talkbass.com/forum/showthread.php?t=352029&highlight=reevesI have spent many hours A/B'ing the Reeves Amplification Custom 225 to the vintage Hiwatt DR201 and the Sunn 2000S while doing helping to finalize the Custom 225 circuit for production. The amp started life as a DR201 clone, the final version actually takes some tonal cues from the legendary 2000S.
We brought the amp to the convention to see if it would pass the scrutiny of some of the most knowledgeable Hiwatt heads in existence (even Dave Reeves son Glynn flew in from England for the event) and it passed with flying colors, or maybe colours.
Score one for Sunn, sort of.
Here's some pics from the weekend. The cabinet we used to compare the vintage DR201 to the Reeves is a '73 Hiwatt 2 X 15, kind of their Fane loaded version of the 2000S cab, but this particular one was JBL equipped. Sounded marvelous. And LOUD!