AES256-ctr was just added in ASA software version 9.1(2). I don't believe the ssh encryption type is configurable in the ASA ssh server. You need to specify it in the client - I did verify it will connect when yo do that (see output below).
SSL encyption ciphers can be specified to exclude the weak ciphersuites.
# sh ssh session det
SSH Session ID : 1
Client IP : <deleted>
Username : <deleted>
SSH Version : 2.0
State : SessionStarted
Inbound Statistics
Encryption : aes256-ctr
HMAC : sha1
Bytes Received : 1824
Outbound Statistics
Encryption : aes256-ctr
HMAC : sha1
Bytes Transmitted : 5632
Rekey Information
Time Remaining (sec) : 3277
Data Remaining (bytes): 996142580
Last Rekey : 07:12:38.807 UTC Tue May 20 2014
Data-Based Rekeys : 0
Time-Based Rekeys : 0