When you say an HTPC machine, I assume you are running something like Kodi and you have video content stored on a drive in or connected to the HTPC box. The thing is that frontend software such as Kodi needs to be able to connect to a backend system that has the PVR backend, unless you have no desire to ever record anything. While you can run the backend software, such as TVHeadend, on the same machine as your frontend, that may or may not be the smartest thing to do. There is a package called OpenElec that (I THINK) contains both the Kodi frontend and the TVHeadend backend. Please check further and don't take my word on that, and in any case you may have to specify that you want TVHeadend during installation. I don't care much for OpenElec myself, because it tries to insulate you from Linux, but some people consider that a feature.
If you are asking is whether there is a standalone box that will act as a backend for your HTPC box, there are a few that I am aware of, they are the
TBS MOI+, the
TBS MOI Pro, and the
TBS MOI Pro - AMD. Be aware that if you go that route, the MOI+ only takes one tuner card so you have to choose between a DVS-S2 card such as the
TBS-6905 or a terrestrial card. Since you're in the US the only choice for that would be the
TBS6704. I would recommend you skip the MOI+ anyway and go with one of the higher end models, not only can they accept more than one card but they are better devices.
Or you can build a backend yourself and install TVHeadEnd on it; chances are that if a box will run Ubuntu Server and will accept PCIe cards it will work and you will be able to run TVHeadend on it, but that requires you to know a little bit about Linux. If you are building a new server, make sure you can return the motherboard if it doesn't work because a few motherboards apparently just won't work with TBS cards. And as I said, it is certainly possible to run both the frontend and backend software on the same machine, but sometimes it's more convenient to have separate systems, particularly if your antenna and/or satellite dish cables come into your home at a spot that's nowhere near your TV's. I'd try to put the server in a place where you can give it direct wired access to your router because some people have reported that they have more problems if they try to use WiFi.
There was a
review of the TBS MOI+ that talked about that device and gave some hints for setting up TVHeadend but it's probably a little out of date by now, particularly in regard to versions of TVHeadend. Then again if you buy one of the MOI devices it might come with an older version of TVHeadend for all I know. I'd check on that, because if you want to watch satellite signals in the USA you probably don't want a version of TVHeadend older than 3.9.2100, and device with embedded firmware can often be pretty far behind in versions of included software if the device manufacturer is slow to include updated versions.
I don't know of any other device besides the MOI devices that may do what I think you want to do, but perhaps someone else will, or maybe I'm not understanding what you want. One reason to build your own backend would be if you want to use some satellite or terrestrial tuner card or device that's not made by TBS, since the MOI's only support TBS cards and even then not every card that TBS has made. That may not be as true of the MOI Pro - AMD, it may at the very least be able to utilize external tuners such as the HDHomeRun devices for receiving ATSC signals, but I just don't know, and I suspect that's a pretty high end (read: pricey) system.
Without knowing what you already have, and exactly what you're talking about when you say you have a HTPC machine (What operating system? What software? Is it a standalone PC or a dedicated device or what?), it's hard to give any more specific suggestions.
