here is the low down;
1. for those speakers (which i've owned recently), your amp will deliver it's 2ohm rated power to them...110 watts rms @ 2ohms.

2. those speakers are designed and engineered at 2ohms (the tweeter is 2ohms, the midbass is 2ohms), therefore they run at 2ohms and you can not change them to 4ohms. The amplifier's power output is determined by it's load (4ohm, 2ohm, 1ohm, etc...). Therefore, the amplifier will see a 2ohm load and deliver it's 2ohm power rating into the speakers.

3. That's an individual opinion. Having owned and listen to 4ohm and 2ohm speakers side by side for years, I personally can't hear a difference in a quiet closed garage at night. In a moving car, I know i can't hear a difference. That's up to you to determine though. I can say that the extra power you can leverage from your amp (given that most amps double their 4ohm power into 2ohms) more than outweighs the slight (if there even is one) SQ improvement, and the extra power may actually improve SQ by allowing better control and less clipping at listening volumes (typically more power sounds better than less power as is obvious with deck powered speakers, and outboard amp powered speakers).

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2004 300M SPECIAL
Alpine CDA 9833
Alpine Pxa-h701
Alpine Rux-c701
Rockford fosgate p400-4
Rockford fosgate p4004
Hifonics BX 1205D
JBL c608 GTi
JBL p963
Infinity 2.5" center channel
Alpine type R 12"(2)
all active SQL set up with 2000 watts rms