Chris, that is not actually what we did.  Personally, I find indexing simplicial
sets by inner product spaces to be unnecessary and unhelpful, and I've not coauthored 
any paper with such a construction.  One can easily compare symmetric spectra in
simplicial sets with symmetric spectra in topological spaces, and one can easily
compare symmetric spectra in topological spaces with orthogonal spectra.  I see
no point in a hybrid.  As a matter of detail, in defining orthogonal spectra one
can perfectly well work with all finite dimensional inner product spaces, without
choosing a universe, whereas the universe is needed to define the linear isometries
operad used in the EKMM construction.  It is nice to keep the S^V as they are: that
makes generalization to G-spectra effortless, where G is a compact Lie group, and
that works for both orthogonal spectra of spaces and EKMM S-modules.  

I prefer eclecticism: the different models have different advantages.  Here is an
eclectic correct definition: a map of symmetric spectra (of spaces) is a weak 
equivalence iff its pushforward map of orthogonal spectra induces an isomorphism
of homotopy groups. (Proven in the paper MMSS Chris cites.)

ps: I really don't like ``if you really must ...''.  There are serious advantages to
working in a model category in which every object is fibrant, and, related to that, 
both for theory and computations it is very helpful to have a clean zeroth space
functor from spectra to highly structured spaces.