I have them too, since 1992 (!), now running them with a Naim Nait XS. Since they are quite aged (caps..) I frequently - but as it turns out prematurely - blame them for harsh treble while after tinkering & swapping cables etc, I identfy other sources of harsh treble, e.g. grounding issues, DAC, Power supply, etc. So overall seemingly quite balanced sound for an oldie, if leaning perhaps a bit to the well lit side. Quite deep in bass, and perhaps a bit light on mid/upper bass (though more than compensated if amp and accessories have a certain emphasis on that region). Note use of specific accessories (cables) can change that impression in any arbitrary direction.
I found speaker stands changed the quality of sound to a very astonishing degree, after I have replaced too tall stands (60cm) with a very massive DIY version of the Linn Kustones in thick steel, top & bottom plates laminated with multiple sheets of wood and centre pillar filled with lots of sand. They would also do a good job as anti tank barriers.
I can say power helps to drive them: I have run them for some long time with an Arcam Alpha 5+ amp, and was quite happy, but then noticed how much better deep and upper bass were balanced when improving on power cables (i.e. replace cheapo distributor, remove long extension lead to distributor - biggest error to avoid!, and also upgrade power cord between distributor and amp). Now with the NAIT XS power is a different league, I assume the relatively high impedance (min 6.something) suits the XS.
Speaker cables: run them on old audioquests two runs of AQ Crystals (each ca 4mm^2) for biwiring. Fatter cables on bass help to emphasise mid/bass over treble, but too much of it consistantly reduces transparency, so that it is better to keep them slightly light (& nimble) on bass.
Bottom line: Since I purchased the Delta 2 Speakers I have replaced every single component in my system at least once, and the only thing that stayed in the chain are the deltas (although I intend to replace them soon, and I feel it shouldn't be too difficult to replace them (given their age) with current, good quality speakers, say Kef R300?).
One more question: has somebody ever tried to pimp them, replace caps, coils, resistors, etc? Any success? any tips & warnings?? feedback appreciated.