Seems that recently Microsoft had abolished all previous 25% off coupons for TechNet Plus, now there are only 15% off ones, like this, TMSAM04.
The only problem these subscriptions have since birth is the strict limit of use. You can pay relatively much less for all the big titles, but you cannot use them as freely as you want to. TechNet sub is limited to evaluation only, while MSDN is for internal test and development.
However, Microsoft is quite a generous corporation. They don't care that much if you "overuse" your subscriptions a little bit.