The Founding Fathers explicitly made the Senate a "house of the States", where Senators, essentially acting as agents of the state legislatures, had the power to amend or veto bills produced in the House of Representatives. However, being unelected, Senators while enjoying greater prestige than Representatives, were also in a position where their powers were not democratically derived. The "check" as it were on the Senate was that any significant interference in bills would inevitably be viewed somewhat more dimly, which is how it has worked out in most Westminster parliaments.
With the 17th Amendment, the Senate gained the democratic legitimacy which in facts leads to the greater possibility of this seeming end-run around the requirement that money bills originate in the House. You don't really find this happening overly much in Canada, where the lack of democratic legitimacy means that Senators usually do not feel they have the right to alter taxation or spending bills. In the UK, of course, explicit measures were put in place in the 1911 and 1949 Parliament Acts that heavily restrict the House of Lords' ability to tamper with such bills.