Have you checked out SoundBocks? https://soundboks.com. Source: about 1 year ago

I had a look at the SoundBoks this weekend at an event I was playing at. It had only a 10ms delay which was pretty awesome. This is out of your price range at 1000 USD, but mentioning here for anyone else looking for wireless portable speakers: Https://soundboks.com/. Source: about 1 year ago

Starting Monday, April 4 at 4PM CET/7AM PST—and repeating every Monday at the same time for the following weeks—we will make a limited number of batteries available for sale on soundboks.com through the regular product page. We’re limiting purchases to only ONE PER CUSTOMER. It will be first-come, first-served, and when the week’s supply is gone—it’s gone. But there will always be the next week :). Source: about 2 years ago

Not sure it has those specific features but you may want to check out Sound Control. Source: 12 months ago

SoundControl is exactly this and more: https://staticz.com/soundcontrol/. Source: about 1 year ago

Hey, bro, I think I've found a solution: I installed the SoundControl software, enabled the "Force Soft Volume Control" option. Yay, the volume is now adjustable! Hope this helps you too! Screenshot. Source: about 1 year ago

I haven’t installed it yet, I’ve got a few days off and don’t want to waste the trial period. This is the one I bookmarked from the ones I found: https://staticz.com/soundcontrol/. Source: over 1 year ago

IDK whether a 3rd party software audio mixer would help; you may download a trial here: https://staticz.com/soundcontrol/. Source: over 1 year ago