I know this is an old post but thought to share the results of my online search after reading the comments over these 5 pages...
The price might not be a sub $200 but something to think about it.
Considering the heating requirements for a 6-6.5 L unit to be at least 300W, there is a DKSonic brand (also DeKang brand that could be related?), model DK-600PF, that has 180W of sonic power and 300W of heating. It's dual frequency with a bunch of other features.
I checked on Aliexpress and Alibaba and it seems that the big problem these days is the shipping cost, over $100. I think, if it wasn't that then that'll be a great price for great specs?!.
I checked Venor.ca and it seems to have decent prices with free shipping. A dual frequency 6L for 257 Canadian. Not bad.
https://vevor.ca/products/6l-digita...steel-lab-0-99mi?_pos=21&_sid=61da0cfcc&_ss=r
I believe that the low frequency in the venor one is 28KHz and the DKSonic is 33KHz. Not a matter expert but I guess the lowest of the two is better. So a DKSonic with 28KHz would have been better.