Most quality speakers (and the Helicon's are certainly that) have the issue of time alignment of drivers incorporated into their design, either by using a sloped baffle or having the tweeter slightly recessed relative to mid/bass divers. In this case, using different length spikes causing a back tilt may do more harm than good. Check the literature on the Dali's to see if the drivers are time aligned - I'd be surprised if not.
Correct imaging
I would like to upgrade the factory spikes on my Helicon�s. I was considering 1.5� spikes, front and rear, from Brass & Granite Audio. However, when I was surfing their website I saw a spike tip which recommended 1.5� spikes on the rear and 2.15� to the front of the speaker to obtain the correct tilt for correct imaging. It is true for all types of speakers or is it design specific? Also is the tilt a function of a distance? Any preferable angle? Thanks for the response!