Summary: I don't have evidence for a 10x increase since the 1960s. I do for an increase from 1.5% THC to 8.8% THC (i.e. fivefold) from 1980-2008.
The University of Mississippi have a long-running "Potency Monitoring Project".
Potency trends of delta9-THC and other cannabinoids in confiscated marijuana from 1980-1997, ElSohly MA, Ross SA, Mehmedic Z, Arafat R, Yi B, Banahan BF 3rd., J Forensic Sci. 2000 Jan;45(1):24-30.
They analysed over 35,000 confiscated "cannabis preparations" over a period of 18 years, and found a rise in potency.
The potency (concentration of delta9-THC) of marijuana samples rose from less than 1.5% in 1980 to approximately 3.3% in 1983 and 1984, then fluctuated around 3% till 1992. Since 1992, the potency of confiscated marijuana samples has continuously risen, going from 3.1% in 1992 to 4.2% in 1997. The average concentration of delta9-THC in all cannabis samples showed a gradual rise from 3% in 1991 to 4.47% in 1997. Hashish and hash oil, on the other hand, showed no specific potency trends. Other major cannabinoids [cannabidiol (CBD), cannabinol (CBN), and cannabichromene (CBC)] showed no significant change in their concentration over the years.
More recently, they updated this to show (now over 46,000 samples):
The data showed an upward trend in the mean D9-tetrahydrocannabinol (D9-THC) content of all confiscated cannabis
preparations, which increased from 3.4% in 1993 to 8.8% in 2008. Hashish potencies did not increase consistently during this period; however, the mean
yearly potency varied from 2.5–9.2% (1993–2003) to 12.0–29.3% (2004–2008). Hash oil potencies also varied considerably during this period
(16.8 ± 16.3%). The increase in cannabis preparation potency is mainly due to the increase in the potency of nondomestic versus domestic samples.
Potency Trends of delta9 THC and Other
Cannabinoids in Confiscated Cannabis
Preparations from 1993 to 2008, Zlatko Mehmedic; Suman Chandra; Desmond Slade;
Heather Denham; Susan Foster Amit S. Patel; Samir A. Ross;
Ikhlas A. Khan; and Mahmoud A. ElSohly. J Forensic Sci, September 2010, Vol. 55, No. 5,