I recently sold my 5i. It only has balanced inputs and outputs which means you'll probably have to buy some XLR/RCA adapters to incorporate it into an HT setup.
I think different people mean different things when they talk about an HT pass-through. I think of it as a pair of preamp inputs for which you can preset the output volume level (as is found on the BAT VK-30SE). Volume for the HT system is then controled by the pre/pro. I've also heard of pass-throughs for which the preamp is essentially a conduit for the signal. The preamp does not boost the signal in any way, and the volume is entirely controled by the pre/pro that is upstream of it. By either definition, the 5i does not have an HT pass-through. However, it can still be incorporated into an HT system the same way any stereo preamp can--you just have to memorize and manually set the volume level each time to keep the volume of the mains proportionally balanced with the other speakers. But I wouldn't call the latter option an HT pass-through.
I have no experience with the other preamps to be able to compare them.
I think different people mean different things when they talk about an HT pass-through. I think of it as a pair of preamp inputs for which you can preset the output volume level (as is found on the BAT VK-30SE). Volume for the HT system is then controled by the pre/pro. I've also heard of pass-throughs for which the preamp is essentially a conduit for the signal. The preamp does not boost the signal in any way, and the volume is entirely controled by the pre/pro that is upstream of it. By either definition, the 5i does not have an HT pass-through. However, it can still be incorporated into an HT system the same way any stereo preamp can--you just have to memorize and manually set the volume level each time to keep the volume of the mains proportionally balanced with the other speakers. But I wouldn't call the latter option an HT pass-through.
I have no experience with the other preamps to be able to compare them.