Hannah,
In yoga, tough can mean two things -
a. tough because the poses are inaccessible to "average" levels of flexibility/balance
b. tough because of the demands they make on endurance/strength using largely accessible poses
Ashtanga yoga is a format that manages to combine both. There are not too many good DVDs I am aware of (since I am not advanced in aspect a.) that can give Swenson's Primary series or beyond a run for their money. But you will find options for Short Forms. T
his is is essentially a disclaimer that I may not quite know what I am talking about !
Shiva Rea's Yoga Shakti has two long practices that you may find interesting because her sequencing and pose selection is different from classic Ashtanga. They fall mostly in category b, although she does some amazing arm balances that may need working up to based on where you are now. This DVD is one of my enduring loves for yoga.
You may alao want to check out Maha Sadhana 2 by Dharma Mittra. There is a clip at the paranamaya site:
http://www.pranamaya.com/mittra-ms2.html You may want to take a peek at Tripshicore Yoga while you are at the paranamaya site (I consider it purely a fascinating spectator sport).
You could check out Mark Blanchard. I dont do his DVDs yet but I hear he fits the bill. Some people complain about his personality though - from what I gather not exactly the humility/personna you come to expect from someone who teaches yoga. People who like a challenge swear by the toughness.
I like Rodney Yee very much, so here is another DVD that I dont have, but think you may enjoy:
http://www.totalfitnessdvds.com/Fitness-DVDs/Gaiam/Advanced/RODNEY-YEE-ADVANCED-YOGA-p6442913.html
I second the suggestion to look at Yoga Challenge 2 and 3 from Tony. For one thing it is a non-vinyasa style so will feel very different from moving from pose-to-pose with Swenson. For another it is based in the Bishnu Ghosh tradition so a very different lineage from Pattabhi Jois who inspired Ashtanga/Power styles. There is a long clip on youtube that combines all three DVDs.