Am I missing something obvious?

My wife has about $120k of student loans. Interest rates are between 5.5% and 6.5%. We have more than enough in a brokerage account to sell assets and pay the loans off, with minimal capital gains. It seems like the obvious thing to do is pay them off, or am I missing something? Other points to note: We have a combined income of ~$300k (so let's assume 35% marginal tax rate), more than 6 months of living expenses in cash, and will continue to max out 401k. We've also agreed to continue to invest what would have been the debt service payments back into the market. Is it silly of me to just think that, say, paying off the a 6% student loans is the same as earning 9% pre-tax, guaranteed?