Try this to help understand more about line weight, and rod designations:
1. AFTM Fly Line Components:
http://www.common-cents.info/aftma.pdf
2. Common Cents System:
http://www.common-cents.info/
It all depends on how far you intend to cast, and what "9 wt" means to that TFO fly rod of yours. It may be that the Orvis rep you talked to knows that a 9wt TFO fly rod is usually closer to a 10 wt. The rod designations (9 wt, 10 wt, light, medium, heavy) are a very broad range and often overlap from one manufacturer to another. It may require some trial and error to find the right match for your application(s).