Was looking at the subject US patent 6,839,541 and see that a company called MISONIMO CHI ACQUISITIONS L.L.C. heavily cites this patent, but I cannot find out what this citing company MISONIMO CHI ACQUISITIONS L.L.C. does. Is it a NPE or what?
Well for one, I wouldn't characterize a company having four patents that cite to the same prior art reference as that company "heavily citing" the prior art reference. See US Pat. No. 4949976 for example.
Regardless, all the citation means is that whoever was prosecuting the four Misonimo patent applications thought the patent examiner would believe the '541 patent might be relevant to the applications' patentability.
Assuming the four Misonimo patents are for technically related inventions (and without checking, I'm guessing they are all from the same patent family), I wouldn't be surprised if each one cites the same, or a very similar, set of prior art references, which happens to include the '541 patent.
As to whether Misonimo Chi Acquisitions LLC is an NPE, your guess is as good as mine. (FWIW, my guess is yes.)
Misonimo Chi Acquisitions appears to have about 8 patents issued, 6 of which issued in the last 3 years. This roughly coincides with their date of incorporation.
My guess is that this is a holding company rather than an NPE. Unlike an NPE, a holding company is usually owned by a parent company that does directly practice the invention. However, this is just my best guess, and I will leave it to others to provide additional information or opinions.