You are most likely wrong about it being low signal, more than likely you have something causing noise on the lines.



A cable modem will work fine with very low signal if the line is clean and there is no noise. After noise the next biggest issue is high upstream, which can be bad wiring, bad connectors, a bad splitter, a loose connector, or any other number of issues.



I deal with it on a daily basis, and while every single customer I have ever dealt with has been adamant that the problem is low signal, I only find low signal at the point of entry to be the problem in rare cases. I remove more amps than I install by about five to one. I do replace a lot of drops, and belly crawl around a lot of crawl spaces and attics looking for bad splitters and tracking down noise.



Have you done any of the wiring in your house yourself? Are any of the splitters in your house gold colored?

Are all eight lines in use? An eight way splitter attenuates the signal about -11dB, which is a shitload. With eight outlets in the house, if they all need to be active I would look hard at what equipment is being used. If your system still has analog and some of the sets have no box then I would look at using a three way splitter, one leg feeding the modem, one feeding a splitter feeding any two way boxes, and the third leg feeding an amp for any analog sets. Of course I have no idea what kind of services they are running on the plant in your area. In my plant we have no analog, our digital signals are QAM256, and we also run video over IP on our newer boxes, so we have learned to chase noise and pay close attention to upstream levels.



Analog picture is far more sensitive to low signal than digital, at least if picture quality is important. An analog image will start to look like crap at -5dB or so, where as a digital box will work fine on -12dB. Digital is where line conditions really come into play, ingress with analog is obvious, you start to hear things that shouldn't be there and see a degradation in the picture, with digital ingress will show up as pixeling and modems dropping.



Currently my modem is running at -12dB, 37 DS SNR and transmitting at 40, it has been online for twenty nine days with zero drops. What that demonstrates is that even though the signal is low at the modem there is no noise on the line, and the transmit is perfect, and those two are the really important issues for a reliable modem.

Have any of your neighbors been doing yard work lately?



Even with underground plant there should be a housebox on the side of your house, normally near the power meter, sometimes there is no box and just a a groundblock with coax going into and out of it, and in old installations there may be splitters screwed directly to the wall in that same area (yeah, they used to roll like that in the old days).



 

That is up to you, my company wants us to keep all inside drops active to make it easier for the customer to add equipment on their own in the future. I suggest to customers that if they don't have plans in the near future of using a line, to leave it disconnected, the more you split a signal the lower the signal gets, the more unused lines you have the greater the chance of one of them being damaged and flooding the rest of the house with noise, sometimes flooding the entire neighborhood with noise.



The trouble with doing that is that I honestly couldn't suggest a place to get a quality four way splitter. All I know is that I replace all gold splitters I see, even the expensive monster cable splitters. Some of them may be fine, but in my experience they are poorly shielded (most cable companies require a splitter to be shielded to -120dB), and cause problems with upstream signal.



 

Bi-directional doesn't mean that the signal is amplified in both directions, it just means that it will allow a return signal to pass. IIRC a two port amp generally creates +12-15dB gain on the forward path and -8dB gain on the return path, which means it increases the downstream signal by a bunch, but also increases the upstream path by a bunch.
The only amps I have ever liked are the Electroline two ports, and the new Evolution five and eight port models with upstream amplification. The eight ways are great, you get +0dB up and down stream. Which means that you can replace several splitters with no upstream or downstream loss.
ETA: As for the splitters, I have no idea why you can't get decent splitters. I have seen a few that I believe came from Lowes or Home Depot that seemed to work fine, but that is about it. Mainly it comes down to shielding and construction. Most splitters available have a very thin cover on the back that is glued shut, the splitters we get have a cover on the back made from the same metal as the body, and it is soldered closed. Once inside the splitter the gold ones generally have several wires wrapped around a magnet, ours actually have a little circuit board inside.





Some of the more expensive splitters readily available have a plastic cover on the back, care to guess how well plastic shields RF?





It isn't even a cost issue, I doubt the cable companies pay a quarter for a two way splitter, of course buying them by the box car brings the unit cost down I imagine. I don't care where your splitter comes from, honestly I don't. What I have found is that most splitters sold at retail will fail any test I can think of brand new out of the box.