If you use a preamp from anyone, be sure you do not use a splitter for TV sets before the power in-line module. The combiner that you are using will be just fine, as you are combining with it, not splitting.
What you really have is:
vhf antenna \\
uhf antenna // combiner ---> common antenna cable ---> TV set
When you add a preamp to the system, be sure you add it after the combiner, but before the TV set (or a splitter add in-line before the TV set.)
As for the ghosting, etc. that is the awesome thing about digital TV, you don't have to worry about that like you do with analog. The things you DO have to worry about are artifacting and pixelation, both of which are caused by too weak a signal.
With the readings you are getting, you can use a weaker preamp than someone who is not getting as strong a signal would need. The reason I would go for a weaker one is that a much stronger one could introduce a front end overload to your set and could be interesting, although most sets can deal with it OK.
Whoops, just looked at RS again and that amp only has a 90 day warranty on it, so may just get another one from another source. DAYUM! As far as Warren goes, they are highly recommended on this forum and @ AVSForums, so just know that I would put them at head of the list compared to buying blind, but would recommend calling and asking about returns, warranties, etc. Some people have found these amps at Lowe's and HDepots near them, but none in my metro area carried parts when I needed them. You may wish to check them out as well, sometimes it makes me feel better to pay more and get it locally so I can return it easier if it doesn't work out, but then that is just me.
Good Luck!
-Mike