How can there be knowledge of unconditioned phenomenon without any knower?
The sutta quoted says Nibbana is an "ayatana" or "base". Therefore, it is a sense object. 

The "knower" of Nibbana is mind-consciousness rather than a "self". This "knower" is a "nama-dhamma", "mental element" or "conditioned element". 

Nibbana itself is the "unconditioned element" ("asankhata dhatu" - MN 115) therefore the Nibbana Element is "unsupported" by any condition. 

"Consciousness" or the "knower" only "knows" Nibbana. The "knower" does not "support" Nibbana. 

In short, the enlightened conditioned consciousness, while living, knows the unconditioned Nibbana. 

When the conditioned consciousness ceases to exist, the unconditioned Nibbana, as an element (rather than as a known sense object) continues to exist. 

The element of Nibbana is eternal &amp; permanent. Knowing consciousness is impermanent &amp; conditioned.