Both the NOAD and the OALD give for tense a negative meaning; check tense on the OALD.
On the NOAD you can find these definitions for tense:
- (esp. of a muscle or someone's body) stretched tight or rigid;
- (of a person) unable to relax because of nervousness, anxiety, or stimulation;
- (of a situation, event, etc.) causing or showing anxiety and nervousness.
As you see, no positive meanings.
Intense, though, means "very strong" so you could use it in a positive way as well.
The Etymology for intense is this one, taken from the NOAD:
Origin - late Middle English: from Old French, or from Latin intensus ‘stretched tightly, strained’ past participle of intendere.
Be careful: The fact that you can use it in a positive way doesn't mean that it means "free of tension" like you said. On the contrary, it gives a strong feeling. It can be negative or positive, yes, but still very strong. On the OALD link above it said "the intense blue of her eyes": It can be a positive feature (as in you were admiring it), but you won't think it's a light blue.
Finally, I want to give you another comparison. The terms are Intense and Intensive, and their difference in usage (still NOAD):
USAGE - Intense and intensive are similar in meaning, but they differ in emphasis.
Intense tends to relate to subjective responses—emotions and how we feel—while intensive tends to relate to objective descriptions. Thus:
an intensive course simply describes the type of course: one that is designed to cover a lot of ground in a short time (for example, by being full-time rather than part-time). On the other hand, in:
the course was intense, the word intense describes how someone felt about the course.