But in any case, it really is a moot point. Section 8 clause 1 of the Constitution gives Congress unlimited scope relating to taxes.
Actually it appears that:
Section. 9.
Clause 4: No Capitation, or other direct, Tax shall be laid, unless in Proportion to the Census or Enumeration herein before directed to be taken. (See Note 7)
Note 7: This Clause has been affected by amendment XVI.
This was the clause superceded by Ammendment XVI, the without apportionment bit.