Best tube mono blocks under $15,000?

Keep your 501s.

Thx for all the input. Seems my amp preamp combo may be serving me well. I think I'll audition the Mc275 C2300 combo and go from there.

In the "swing for the fence" category of tube amps, consider buying a used VTL Reference level Wotan MB-1250 Reference designed to "take control of the most demanding loudspeaker load and thunder out deep bass with the authority of a mighty ruler, while at the same time handling the delicate passages with speed and nimbleness of a youthful hero...the Wotan is probably the most powerful tube amplifier in continuous production in the world high-end audio industry. An awesome demonstration of the Wotan's unsurpassed power was setup for Hi Fi '98 in Los Angeles where we used 3 pairs of Wotans in a triamped system with the Martin-Logan Statement speaker systems." (specs)

For something a little more realistic -VTL's MB-450. The MB-450(specs) has "adjustable Damping Factor feedback control"

Just in case, there is a used pair of Wotan 1250($12,000) for sale here right now and two sets of MB-450 Sigs ($4,300 & $10,000)

I'm fairly certain the $4,300 MB-450 Sigs would fulfill your every need and save you ten grand at the same time. Read the many reviews. If you feel the urge for the ultimate in high-power tube amplification, unleash the Wotan. The term "effortless" would certainly apply in your system although a larger air conditioner may be needed.