Assigned an airspeed of 160KTS after being cleared for an ILS approach; the flight crew of a CRJ is called to task by ATC for slowing to SOP approach speeds at the outer marker.

Narrative

We were given approach clearance to join on '240 heading; maintain 6;000 until ZZZZZ; cleared ILS XXL.' I read back clearance. Next transmission from approach was not completely heard; I heard '... contact tower'. I read back tower frequency; and said 'say again on rest'. ATC: 'Airspeed 160 KTS'. I read back '160 KTS'. The First Officer joined approach; flew approximately 5 more miles and configured for approach with landing flaps; and slowed to 140 KTS. Tower cleared us to land; and pointed out traffic ahead was 4 miles away. Approximately 3 miles from the OM; the Final Monitor asked us our speed; to which I replied '140 KTS'. We were then asked what airspeed we were assigned to fly to the OM. Confused at the question; I asked what speed they wanted us at. We were instructed to speed up to 160 KTS to the OM; and we immediately complied. We were notified to call the tower on landing; due to a possible pilot deviation. I contacted a Supervisor of the tower upon entering the terminal. He had already reviewed the ATC recordings; and determined that we were assigned 160KTS; but with no clearance limit. I asked him what the expectations were regarding that type of airspeed limit; and he agreed it was ambiguous; but normally the fix is attached when instructed. The ATC expectation was that we would hold the speed to the OM; and that if we desired to slow; to request that from a controller. He confirmed that no loss of aircraft separation occurred. Clarify ambiguous ATC instructions; we did not intentionally disregard ATC instructions; nor error in an unsafe way; which frustrates the situation. The big questions that surfaces is; 'should we be flying faster than SOP to the OM' on any approach?