This is personal opinion, as audio is highly subjective. I only use tweeters with textile domes (usually silk), which includes component setups by Infinity, older Rockford stuff, and some other brands. I stay away from all metallic tweeters as they are all too "tinny" and "bright" for me. All of my customers were very pleased with my installs which tended to emphasize more cleanliness of the music than loudness.

As for the specs to look for, impedance is going to be the only important one and is going to depend on your current setup (factory DSP, base, etc) and how you are ultimately going to drive them... aftermarket amp, etc.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)