In terms of being the most efficient miner, S9 is at the top but in terms of reliability, I can't say the same (same goes to all the other Bitmain products) and to add to that, there's a lot of bad reviews regarding the repair situation by them. A7 could be another option, although less efficient.
If I were you, I would have spent my money on getting 3 to 4 more high-end GPU's (to intensify the GPU farm that you already have) and just mine altcoins (but that's just me).
Yes the only reason I don't want to reinvest in GPU miningis due to the POS from Ethereum and the AMD vega coming out this Q2.
This is kinda the reason why I wanted to reinvest in mining but in BTC to diversify the mining income.
I personally love the idea of getting monthly passive income
thank you guys for the info!