Hi
I just got my pro80's today after a listening session in a local store, had similar dilema because the other headphones I compared were all priced below the Takstars, even after a roughly $15 discount I was offered on those (which still resulted in them being slightly more expensive than on Amazon, given the prices here in Poland). Among the cans I tested also were Shure SRH240, some Soundmagic, Takstar TS 670, and HD 440 & 681 Evo by Superlux. I have not heard Senn HD439 so I have no direct comparison that you're looking for, but I can share my views on pro80's.
Their sound is by far the deepest and best defined, with the best portraying of all models I tried today (and some of them are considered good bang for the buck themselves), it's surprisingly good for $70 cans, has a rich and profound if sometimes a little overflowing bass (still very realistic and well detailed), natural mids and the treble pronounced enough but not extra sharp which gives a comfortable listen. They are the fullest sounding of ones I considered, maybe not the most neutral you can get but I don't think it could be much better given the price range.
With their circumaural construction, they are comfortable (been listening for few hours now and there's no sign of tiredness as it would be in my case with I think any supraaurals). I wouldn't say the HM5 pads are a must (I tested the Takstars with stocks as well as HM5 replacement available in the store), nonetheless offering a nice extension in comfort (it goes from good to great) and sound presentation, which might have been subjective but I felt that the soundscape gains a bit of depth with them. Noise isolation is very good as with most circumaurals, and is also slightly improved with HM5 earpads.
Overall, I don't know about the Senn's, but I think with closed cans at this price tag you can't go wrong with pro80's sound- and comfort- wise. I don't know about their durability cause I bought them today, but they look and feel rather sturdy, much more so than the Superlux budget cans, if that's a big issue, I would risk to say quite on par with some Sennheisers I had in the past (HD 202, HD 238, HD 25-1 II). Of course, looks can be deceiving so I'll see with time how they endure.
