S-t! That is something. I have a single 18" sub from Velodyne (their DD18) which i have taken a few years to dial-in properly over time with isolation, etc... i can only imagine what 2 21" can do when set up well...yeesh! Enjoy! What is the rest of your system?
I was actually asking what will be connected to it on the other end. You ask for something that will not have "any negative effects on the stereo sound" but that will depend on the rest of the system. Are we talking analog or digital?
Yes there are many, but like Kal said it would be best if we knew the full system context so that we can advise on the best device.
You mention bass management, does this mean you want to actively redirect bass from your main speakers to subs? Or are you just looking for a room correction device for the sub only? How is the sub then being integrated with the two channel speakers?
If however, you are using a full range loudspeaker, you can always resort to a high quality multi-band analog EQ device, of which there are many.
As an alternative, you can take the plunge and commit to a full range DSP correction system, of which there are several, and which can be essentially divided into hardware based solutions (i.e TacT, Lyngdorf, DEQX) or software based solutions (i.e. Dirac live, Acourate). DSP solutions never fails to generate a hotly contested response among audiophiles. There is one school (who considers themselves "purists") that say they would rather eat poison than tamper with the pristine purity of their analog signal, warts and all. Then there are the rest of us (including virtually all recording engineers and record producers) who believe that a well-executed DSP solution might afford a favorable balance between the potential benefits and liabilities of such an approach such that it offers a better overall result than any other alternative for a given system. The key here, is the term "well-executed". DSP, if not done right, can be disappointing. However, as many will attest, it can also achieve spectacular results that can make you smile when listening to DSP done right.