huh?
They are using the same frequencies for digital as analog (albeigh less channel options) so the same amplifier you used for analog will work for digital.
DTV reception requires a very flat bandpass across each 6Mhz channel. Multipath of any sort is very destructive to reception. Leaky cable, poor bandwidth of splitters or cable amplifiers will cause exactly what landtuna mentioned in his post. Also, having too much receive antenna gain too close to the TX site will cause what appears to be, low signal.
The 'signal strength' on the TV set up menu doesn't measure signal level, but signal quality.