ARC tubes v. other tubes

I am new to tube amps and have had an ARC VT-100 Mk.3 for about 3 mos. now. I LOVE it! and have never heard my Genesis 500's sound better. Reading threads and posts here makes me wonder what is the deal w/the stock tubes ARC gives you in their amps and preamps? I also have the LS-15 and PH-3 and bought tubes for retubing in Jan. from ARC. I wonder if I should have looked around for other kinds of tubes? Should I definitely do so when it's time to retube my VT100? I have heard of The Tube Store.com but wouldn't know how to begin to pick other tubes for my equipment. I have in the past just trusted ARC but now I wonder if I could do better.
Thanks for any info and suggestions.