Replacing the power cord with something better on the Storm III will tighten up the bass. The Stratus probably sounds tighter on a quick comparison because it does not go as low, so it does not set off room effects as much. Each REL going up the line is an improvement over the one below it from my experience and from what I have read.
There was someone writing reviews on Audioreview.com that made the same comments about the Storm III and Stadium III not being as good as the Stratus III because of the port issue. I do not agree or believe it, especially considering they feel the Stratus III is a better sub than the Stadium III. But I guess everyone is entitled to their opinion. The Stratus is a great sub; but if you can swing the extra $500 for the Storm, I recommend you do so.
There are reviews on both the Storm III and Stratus III at www.hifichoice.co.uk ---- They liked the Stratus, but felt it did not go low enough. They rated the Storm III their favorite of all the subs they have encoutered. They are a mid-fi and budget audiophile magazine. Mostly stuff under $2000 per piece. I've always liked this magazine. They do not seem to kiss up to companies that advertise with them as some audio magazines do.