I have been using the DE xm 250 10Ks on a PFO HQI ballast for about 2 months. I really like the color and all the frags seem to be holding thier color well under them. I haven't noticed any yellowing that some say they have experienced, though mine are still quite young and still give off a nice crisp white leaning toward blue.
I had a poll running on reefs.org about bulb replacements and the XM 250watt 10,000k/20,000k bulbs topped the list. Nothing but rave reviews over there. I will purchase 2 replacemnt 20,000kXM bulbs next wednesday on payday...
I replace my 250 W Ushio's with XM 10 K's, running on IceCap e-ballasts. Had difficulty getting them to fire consistently, so after 3 weeks or so of "dealing" with them and communicating with the fine folks at Hellolights, the decision was made to exchange them. Now I'm running an AB and an Ushio and no problems what so ever. Would have liked to have save the ~$30/bulb, but I like it more when the lights actually come on.
My suggestion is to check with the sponsors of this forum. Please do not provide prices and such as it is considered a commercial post for those retailers you are talking about ... contact our Reef Frontiers sponsors and I am sure they can provide you with very competitive pricing.
We're burning XM 20K DE on PFO 'HQI' ballasts and getting phenominal results. They have a nice crisp blue look after burnt for a week or so. If they last they'll be awesome Many people like the 10K version and report lamp life in excess of 18 months. HTH
Chuck, the 10k and 20k lamps are great IMO. I run 20k XM's and at first it was blue, then after 2 weeks it has gotten to a point where the balance is just "right" for me. I ran it on 29 gallons using 20k XM 250 Watt without actinic supplementation. The 10k XM's I believe put out more PAR. My 20k XM is going on at 5 months right now, still looking nice.