Yes, but if they are affiliate links then the Hubpages TOS specifies a maximum of two.
Meibel said - "Yes, but it lowers the value of your hub." - I am not sure what she meant by "value", but if she meant hubscore then I would not worry about this if I were you.
If she means the overall value, i.e. how valuable it is to readers then I would dispute this.
If she means the value as placed on it by Google for instance then I would definitely disagree. Referring to authoritative sites can have a positive impact on your hubs relevance according to Google.
Basically, put as many reference sites in your hub as is sensible, unless they are ones you are affiliated to, in which case only two.
If you are placing links to your own websites then this should give some Google love to them as Hubpages is a PR5 (I think) site and this will be a good back-link for them.