If I get a true preamp, that renders the oppo DAC useless right?Not at all! DACs are often inputs to a preamp.
Its common when a source is trying to drive an input that is too low for the source, that the result will be a loss of bass impact. One reason for this can be a coupling capacitor at the output of the source; there is a timing constant with relation to the input of the amp that can cause a low frequency rolloff as a result. So its an engineering thing.
You can just put in a bigger cap to sort it out either- the limit might be distortion by the source trying to drive a lower input impedance.
At any rate a preamp is a good idea- or get an amp with a higher input impedance, or do something else like a zero gain buffer that has a lower output impedance. If you ever plan to have sources other than the Oppo, a preamp is a good idea.