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"Architectual" (man-made) joists have some decided advantages over natural lumber, including excellent unit-to-unit consistency.  Most residential "TJI" joists (as opposed to beams) are made straight and installed without an initial "crown" (manufactured beams often start with a pre-load crown), and TJIs are designed to stay flat under design loads once the subfloor diaphragm is glued and nailed or screwed down to schedule.  In this case, your hi-fi equipment probably exceeds the floor's design loads, so the use of shores in this case is at least pre-emptive structural reinforcement.  As I described earlier in the thread, the usual course is to shore up beams that run perpendicular to and in turn support the floor joists, to divide the floor load, as opposed to using jacks helter-skelter under individual joists.  In any case, the jacks are only truly effective if they are "grounded" to something appropriately solid and stable, and a 4" basement slab is generally considered inadequate for this purpose.  One way around this is to increase the number of jacks and so distribute the load(s).  Another is to use "sleepers" (basically, beams lying on the floor).

I can't think of a structural reason to use the jacks to crank a "crown" into the floor at this point.  You can buy or rent a spinning laser level to check the floor height at any number of "stations"; just do not assume it will in any case be truly "level", nor would one expect it to be.

If you want to "tune" the floor, then you will simply have to use your ears and gut to determine how you like it.  You have probably noticed by now that multiple jacks in various locations can be used to change the "feel" of the room as much as it can alter the "sound", and this "sense" of the room seems to carry up into frequencies that one might say are "not affected" by the treatment.  Don't forget the "wide bowl of water" trick.

All bets are off until you factor in true, strong (U)LF information.

Best regards,
Paul S

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