Volume and Connection Questions

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I do not get the volume that I would like to. Sometimes I have to really crank up the 300 watt Yamaha mixer to do this but then many times I risk getting mic feedback (yes I have the computers volume control all the way up). I am running a standard 1/8 mini plug from my Soundblaster Audigy 2z to a 1/8 mini to 1/4 male connector and plugging it into Yamaha 88s Super Hi-Z connection. This is where I seem to get the best sound quality and the most volume. I am wondering what others are doing.
Is there a better sound card?
Would I be better off running a 1/8 mini to the XLR connection (I have never tried this).
Is my volume limited by the amount of power my computer can put out? Does the power supply have any effect. I have a 150 to 250 max power supply. Here are so other questions I have. Do all mixers/speakers put out some hum? I don't have a lot and I don't believe this is your typical hum cause by a loop feedback because all my equipment is running thru a Monster Cable HTS 2000 line conditioner.
WHAT GAUGE AND BRAND SPEAKER WIRE DO YOU FIND IS BEST
Thanks for any suggestions or information.
Monarch Customer built P4 computer
Hoster
Windows XP Pro
Enermax EG26s power supply
Asus P4R800-VM motherboard
1gb RAM
Western Digital wd2000jd 200 gb hard srive (sata)
Creative Audigy 24 bit 2ZS sound card
ATI (Original) Radeon 9600 XT 128MB graphics Card
Plextor 52x32x52 CD-RW Drive
Yamaha EM88s 250-400 watt power supply
Yamaha Series s115 club series speakers
Peavey Unpower floor monitor
Sure SM58 Mic's
Horizon 14 gauge 30 ft. speaker cables