First of all (as always): do not use the two-sided HP filter for estimating DSGE models. See Pfeifer (2013): “A Guide to Specifying Observation Equations for the Estimation of DSGE Models". If you must, use the one-sided HP filter
Depending on the filter you use, your data will be mean 0 (true for the one-sided and two-sided HP filter). Prefiltering in this case is not needed, because the empirical mean you subtract will be 0. Thus prefilter=1 will be harmless, but useless.
As you mentioned, I used one-sided HP-filter and was curious about whether demeaning is needed.
According to your comment, one-sided HP-filtered data is already mean 0. Then, prefilter=1 becomes useless.
Actually, it is a bit more complicated. With the one-sided HP filter, the mean will only be asymptotically 0. Thus, if your sample is short and depending on your view of the world, you still may want to demean. See also