The VTF-MK3 is a great sub and you should keep in the HT room, there is no reason to change. For the workout room since you just want music you can get a PA sub that are perfect for music, like this -
https://www.sweetwater.com/store/detail/B1200DPRO--behringer-eurolive-b1200d-pro-500w-12-inch-powered-subwoofer.
Keep the Denon in the main HT room as well. For the music room get any old receiver from ac4l.com for <$200.
The workout room should just be 2.1. Music doesn't need center channel.
btw the SVS subwoofers are overpriced and I would never get the Elac subs. If you really want to add another sub to the main room, you can look into another HSU, or look at brands like Rhytmik, Outlaw.