Sub Integration Question / Recommendation

In the process of changing out some cables and upgrading my system, I now need to get Hsu subwoofer dialed in again. The prospect of going through this exercise was enough of a disincentive to making these types of changes before now.

According to previous forum posts, my chosen approach is termed "Bass Augmentation" as opposed to "Bass Replacement". However, the term "augmentation" may be somewhat of a misnomer; that is, my goal is to have the subwoofer "blended" seamlessly with speakers running at full range in a 2-channel application. There was some discussion of “level matching”, but no step-by-step description on how to do it.

Rather than taking measurements (with RS analog SPL meter) above and below manufacturer’s recommended level of 50Hz, I took another route. My speakers go down to 35Hz, and I got a suggested cross-over of 40Hz. So this is the setting used on the sub, with the cross-over switch set to “In”, and Volume at 9:00 o’clock.

Here are the steps I took: 1) Set the main system volume to near the maximum that I generally listen; 2) using Stereophile test CD at 40Hz tone, took SPL reading of main speakers from listening position, without subwoofer (sub turned off); 3) with main speakers turned “off” and again using test CD at 40Hz, adjusted sub volume to match same level of mains measured in Step #2 … in this case 67 db.

When both the mains and the sub were all turned on, something unexpected happened: the SPL level dropped down to just below 64 db. I had expected it to be the same or higher. What’s going on there? Now “theoretically”, is this the correct method to perform “level matching”, or should the sub volume (with mains “on”) be adjusted upwards to reach the same level reached in Steps #2 and #3 … which was 67 db?

I mentioned “theoretically” because - in the long run - I will play around with the settings to fine tune the sound by ear, in order to get it to my liking. That’s what happened last time out, and it’s also the way I got my second system dialed in.

That said, I would still like to know: what is the accepted and correct way to perform “level matching” between main speakers and sub woofer? One would think this should form the basis of the most objective starting reference point. What’s your take and/or recommendation?