Yes, by using an amplifier, you can lower the loading caused by the
detector. There is an optimum load impedance for the detector output
in terms of best output power for a given input signal, and for small
signals it's a pretty high impedance (resistance), because the diode's
dynamic impedance is quite high for very small signals. You can find
info on this at the Agilent web site (at least till Agilent sells off
their semiconductor business...). Look for ap notes and data sheets
covering zero-bias detector diodes. It gets a bit technical. But the
optimum load is, as a rule, rather high resistance. An FET-input
amplifier chosen for low input voltage noise is probably ideal.
HOWEVER, the crystal radio purists would probably complain that it's
not a crystal radio then. As Bill says, a matching transformer can
help you out a lot. I've done some work using zero-bias Schottky
detector diodes driving DC amplifiers to look for small signals, and
can detect signals down in the few tens of microvolts---but the output
is in the vicinity of a microvolt at picoamp currents.

The other thing that lowers the loaded Q of the coil is coupling to the
antenna. Remember, the antenna looks like some impedance. A resonant
antenna looks like a resistance, and an antenna coupler or tuner will
make a non-resonant antenna look resistive also. And that resistance,
coupled to the tank coil in your crystal radio, will lower the Q. If
you couple too lightly, you won't get all the signal you can, and if
you couple too heavily, you will lower the Q so much that you won't get
the desired selectivity. It's a balancing act. In fact, in a
multiple-resonator tuner, the bandpass shape is adjusted by changing
the coupling from one resonator to the next, which changes the loaded Q
of each resonator. When you have but one resonator, you just change
the bandwidth (and signal level) as you change the coupling and loaded
Q. Coupling that's too light mostly just changes the signal level,
with minimal change in bandwidth. Coupling that's too tight mostly
changes the bandwidth, with minimal change in signal level.

Hope these thoughts help some...

Cheers,
Tom