IMO, the only double bells that would sound reasonable in a modern concert band are the late vintage (1930's and 1940's) C.G. Conn 30I or 36I models. These were essentially 4-valve Artist horns with the double-bell attachment. These were the horn typicaly used by the D.C. military bands (in satin silver, gold-wash bells, and fancy engraving) just prior to their converting to Bessons in the 40' and 50's. The sound on the large bell would be similar to my 1968 Connstellation.
A couple of words about the sound and playability of my 1941 Holton. It is in superb original unrestored condition. The valves have never been replated and compression is good on all 5 valves. While this horn may not play as good as when it left Elkhorn, Wis. in 1941, it is darn close.
The small side does sound like a Bb Alto horn rather than a trombone. I suspect the sound is even smaller than a modern english baritone. This is typical of all double-bells since these horns are not AS cylindrical as a trombone, have larger bores, but very small bells (about 6"). They do not resonate like a euphonium nor do they project well like a trombone. So while it can blend with a 'bone section, it cannot replace a trombone on an exposed part. On my horn, the small side actually plays well in tune, but I have to pull the 5th valve tuning slide out all the way to match pitch with the Euphonium side of the horn (which tends to be flat anyway).
The large side of the horn has a few WAY flat notes which apparently are fairly typical of Holtons (low Bb, A, and Ab). These same notes are pretty well in tune on the small side (go figure). The tone of the Euphonium side is thin compared with a modern horn, even though mine is an upright bell. This is to be expected just by looking at the bell section that has a smaller bottom bow and a much later flare than on my Besson Euphonium (or even than on either of my Conns). I can darken the horn up by playing a Wick 4AY but it really hurts intonation (which is problematic to begin with) and makes the high range hard work. This horn plays best with a Bach 6 1/2AL or smaller. Then the high range sings and the horn plays better in tune. However, as Rick points out, it doesn't blend with modern "English-style" Euphoniums (most of which are now made in Germany or Switzerland anyway!)
It is good to hear there is some classical literature written for the horn, but my suspicion is that these tended to be used for their novelty effect by the band soloists of the first half of the 20th century or as doubling instruments in ensembles.
Doug