As I said in the parentheses, the R and XC both decrease but the Q is the ratio XC/R, which remains unchanged even if more caps are put in parallel. The same thing applies to inductor's component Q, XL/R. The main point is that it is very tricky to build a very low impedance tank circuit for power amp and keep its loss reasonably low enough so that the heat dissipation is manageable.
Ryuji Suzuki AB1WX