First answer these questions: Do you want an external or internal frame design, what price range, volume size, and maximum load you plan on carrying.
For external frame packs, check out
[url]http://www.jansport.com[/url]
For commercial internal frame packs, it's hard to beat Arcteryx, Gregory, or Dana Design. If you want absolute comfort, fit, fabric choices, and color choices, try looking into a custom made pack by Mchale. Also, both Dana and Gregory offer modular add on attachments you speak of.
[url]www.danadesign.com[/url]
[url]www.arcteryx.com[/url]
[url]http://www.gregorypacks.com/[/url]
[url]http://www.mchalepacks.com/[/url]
Bushmaster-M4gery: whatever pack you choose be sure to check out Mountain Equipment Coop at: http://www.mec.ca
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I agree, especially true if you're buying Arcteryx products since it is made in Canada - your dollar buys a lot more over there.